The story takes place in Spain, and the Spanish spoken by the villagers in the game is called Castilian (which has a different pronunciation than the standard form of Spanish, known as Español, commonly spoken by many Latino). The currency used throughout the game is the Peseta, which some may find curious, as Spain had switched to Euros in 2002, and the game takes place in 2004.

This is the first Resident Evil game where Umbrella isn't responsible for the monsters and bio organic weapons (BOW) creations. Umbrella is gone by this point, and may yet come back.

This Resident Evil used all new enemies that were not in any of the previous games. No zombies, no Cerberus, no crows, no giant spiders, no tyrants, no hunters, no lickers made it to this installment.

Ashley will cover her skirt and yell, 'Oh, you pervert!' if the player aims upwards towards her while on lower ground.

The last Resident Evil entry directed by series creator Shinji Mikami. He was not involved in the next game, Resident Evil 5 (2009), which he said was not "the game he would have made". He explained that he wasn't especially happy or sad about it, and commended Capcom's hard work, but stated that he simply didn't like the idea of a Resident Evil game that was so different to his style.

This is the first Resident Evil presented in 16:9 widescreen. All previous entries were displayed in 4:3 Fullscreen. This is why when the game is played on an 4:3 television it appears to be letterboxed.

The Matilda handgun was named after the character with the same name from Luc Besson's movie Léon: The Professional (1994), which also had a main character called Leon. The game also contains an unlockable "Professional" difficulty in homage to that movie.

The earliest version of the game was announced for the PlayStation 2 under Resident Evil 2 (1998) (Resident Evil 2) director Hideki Kamiya. This build was eventually canceled as a Resident Evil installment, and became the title known as Devil May Cry (2001). Eventually, development was moved to the GameCube console and another build emerged, produced by Shinji Mikami. This was the first version to feature Leon from Resident Evil 2 (1998), sporting a variation of his current costume and fighting a mist-like virus. The second shown version had one bad guy in particular that sported a hook on one of its hands, and the final logo for the game still reflects this. A third version was created but never unveiled to the public; it contained the classic zombies again, but it was canceled after a few months, before Mikami took over as director and transformed the title into its final version. In Japan, an exclusive DVD bonus disc included with the game had footage from the second version that was made.

Resident Evil 4 was in production for many years due to the constant changes it went through. The game was scrapped and started again four times. The first scrapped version would have featured a character called Tony Redgrave, the son of Ozwell Spencer mentioned in Resident Evil (1996) and Resident Evil (2002), who gets superpowers from being infected with the G-virus. However, producer Shinji Mikami felt that the concept strayed too far from the series' survival horror roots. This version eventually became its own game (and franchise), titled Devil May Cry (2001), with Tony changed to Dante.

The hallway sequence where the player has to dodge lasers via quick time events, is likely a reference to the Resident Evil (2002) movie, where the film's characters encounter almost the exact same situation.

The Killer 7 semi-automatic Magnum was named after Killer7 (2005), another Capcom game produced by Shinji Mikami. This is the only weapon in the game not to feature an exclusive upgrade.

The gameplay was changed after series creator Shinji Mikami had been working on Resident Evil Zero (2002), and realized that many members of his design team were getting tired and bored of using the same old survival horror formula from other Resident Evil games. This convinced him that Resident Evil 4 needed to be taken into a wholly different direction, both in terms of story and gameplay.

All cutscenes in the game were pre-rendered, which can cause some continuity errors that only occur depending on the choices and settings the player makes. For example, during any cutscene in which Leon draws his gun, it will always be the same one from the beginning, even if it had been sold and replaced with a different gun. And although Leon's and Ashley's outfits can be changed in the game, they won't show in the cutscenes for the same reason.

The original GameCube version contains a unique glitch where sometimes after reopening a save file, Leon will move sideways instead of forward. If you reset the game and reopen the save file though, Leon will walk straight again. Another glitch caused Leon to move much faster than normal, making shooting and slicing with a knife much easier.

An HD remaster of Resident Evil 4 was released for the Xbox 360 and PlayStation 3 in 2011. An Xbox One and PlayStation 4 remaster was announced on February 25th, 2016, and was released on August 30th of the same year. This remaster ended up just being a port of the PC's Ultimate HD Edition to consoles. The PlayStation 4 and Xbox One versions render in 1080p at 60 frames per second, features the updated textures and character models from the PC's Ultimate HD Edition, and include all bonus content from all previous versions of the game. There is no option to use the original GameCube assets though, unlike the PC's Ultimate HD Edition. This version was finally released on the Nintendo Switch in 2019 as well, without the Switch motion control function, though.

Strangely, there are no children seen among the villagers in the game. Originally there would be, but due to strict rules concerning the depiction of violence targeted at children in video games, the developers decided not to include them. Although not addressed within the game, the official explanation is that children cannot survive the Las Plagas infection, so they have all long died before Leon arrives at the village.

The backstory between Leon Kennedy and Jack Krauser is partially explored in Resident Evil: The Darkside Chronicles (2009) where they were fighting partners in Operation Javier.

This is the first Resident Evil game to feature Pro Logic II surround sound.

During the main game's end credits, pictures are shown of the villagers before and during a Las Plagas infection. Apparently, these pictures come from a book that Ada Wong can be seen holding in the house of Bitores Mendez, in the beginning of Mission 2 of her 'Separate Ways' game.

The music heard during chapter 2.2 as Leon and Ashley try to escape the rural village at night may be familiar to those who have played Resident Evil - Code: Veronica (2000); it is the same music heard when Claire Redfield enters the torture basement behind the guillotine at the prison facility.

The first Resident Evil game to not use an inventory system consisting of a number of slots that can hold one item each. Instead, there are briefcases with a grid system that allows the player to store items more efficiently, based on the available storage space rather than predefined slots. The cases can be upgraded to larger ones throughout the game to allow for even more storage space.

Those infected and controlled by Las Plagas are called Ganados. In Spanish, "ganado" means "cattle" or "livestock".

Two different versions were made for the PC. The first version was based on the PlayStation 2 version and published by Ubisoft in 2007. It was widely criticized for the lack of mouse control, missing lighting in certain parts and the low texture resolution, which made it inferior to the PlayStation, Wii and GameCube version. A vastly improved second port, which was based on the HD versions for the PlayStation 3/Xbox 360, was published by Capcom in 2014.

Concept art reveals several types of enemies that didn't make it into the finished game. These include: the 'Electric man'; the Parasite Woman; and the Lurching Man.

The cult name Los Illuminatos translates to "The Enlighted" in English.

The second GameCube version of the game that contained Leon Kennedy was called the 'Hallucination version' or 'Hook Man version', taking inspiration from the horror movie Lost Souls (2000). It kept the haunted European castle setting and the premise of Leon looking for an unknown girl from the previous 'Castle' version, but abandoned the fog-like virus in favor of imagined monsters. Since this version was scrapped before a full script was written, few other story details were revealed, apart from Umbrella's Progenitor Virus (the basis for Umbrella's T- and G-viruses) infecting Leon, who then starts to have hallucinations where dolls and other objects come to life, and he has to fight an enemy called the Hook Man.

A fan game was being developed by a Resident Evil fan who goes by the name ShiguWorks, the project was called "Resident Evil CODE: Madman", which recreated the canceled "Castle" and "Hook Man" versions of Resident Evil 4 (2005).

Unusual for a horror game at the time, this one was released uncensored in Germany, where it obtained an 18 rating. However, the modes "Assignment Ada" and "The Mercenaries" were not included in the German version. This was because Capcom was sure that once the main game got the rating, it wasn't necessary to submit those modes for review. The complete version of the game that contains all modes was therefore banned in Germany in 2005 (the ban was lifted in 2016). After the release of the German GameCube version, Capcom later noticed that both modes were required in order for the player to unlock the Hand cannon and the Chicago Typewriter. When the Wii version was about to be released in Germany, Capcom modified the game to make it possible for the player to unlock both weapons once the game was completed on the Professional difficulty mode.

The parasites, Las Plagas, translates to "the plagues" in English.

This is the first game in the series in which the iconic zombies don't appear. Replaced by Plaga infected Ganados.

If you save the dog from the bear trap at the beginning of the game, it will come back to help you during the fight against El Gigante.

Hidden cutscene: before the battle with the lake monster, if Leon shoots at the water from the pontoon, the monster will come out of the water and eat him.

At the start of episode 1-3, after Leon's second encounter with the bearded chief Mendez, if the player chooses to go back in the bedroom to follow him, it will trigger a hidden cutscene. The chief violently attacks Leon, but he is rescued in time by an unidentified Ada Wong. The same cutscene appears in Ada's 'Separate Ways' side-mission.

After the fight with the lake monster, there is a short hidden cutscene with wolves if the player goes back to the swamp area. If the player follows the normal path, wolves are not encountered until Leon gets back to the church.

During the end boss battle, Ada Wong suddenly appears and throws a special rocket launcher that can be used to kill the boss. The same thing happens at the end of Resident Evil 2 (1998).

The first port of the game for the PlayStation 2 contained an additional campaign called 'Separate Ways', in which the perspective of Ada Wong is followed in parallel with the events of the main game. This side-mission has made it to all subsequent ports, re-masters and re-releases, and explains several plot points in the normal game: how Ada rang the church bells that drew away the villagers from Leon in the beginning, and how she rushed to come to his aid several times despite orders to kill him. Also, Leon's coat that disappears after he is first taken prisoner can later be seen worn by a villager in this scenario. 

The confrontation with Verdugo (Salazar's 'Right Hand') is the only boss fight in the game that can be partially skipped. The player will need to fight or avoid him until the elevator is activated, and can leave afterwards; Verdugo will not follow below. However, if Leon returns to the area, Verdugo will still be waiting for him.

Albert Wesker, long-time nemesis in the Resident Evil series, is only mentioned in the main game as the mastermind behind Krauser's and Ada's involvement in obtaining a Las Plagas sample. He makes a visual appearance in the 'Separate Ways' side-missions, though.

Hidden death: if you hit Luis five times during the Ganados ambush in the cabin in chapter 2-2, he will shoot and kill you.

When Ada and Leon meet up in Salazar's quarters, Ada sneaks up from behind and (after a brief fight) tells him: "Long time no see". Albert Wesker says the same thing when he approaches Chris Redfield from behind in Resident Evil - Code: Veronica (2000).

Nuby Tech designed a unique controller for the game in the form of a yellow chainsaw with gory blood marks. It comes with a built-in sound chip, imitating the roar.

For the Windows version, the background of the game's main menu crawls slowly. Pressing left or right on the analog stick controlling the camera causes the background to scroll faster in a panorama like fashion in the direction of the stick.

In the US version, if Leon is attacked by one of the chainsaw-wielding villagers, his head is chopped off. However, in the Japanese version, when a villager attacks Leon with the chainsaw, it cuts off part of Leon's face, exposing half of the front of Leon's skull. It graphically shows Leon's right eye still intact as well, surrounded by blood. While this is more detailed and gory than the US version, the US version is more realistic in terms of anatomical surgery. Additionally there are no exploding heads after heads shots and the physics of Ashley's breasts were removed.

The Broken Butterfly gun has the description, "A very powerful .45 magnum revolver. This will make anyone's day." Most likely this is a reference to the 1971 movie Dirty Harry, where the title character wields a magnum revolver and says the line, "Go ahead, make my day."

Mistakes

During the scene where an armed helicopter assists Leon in cleaning out the bad guys, the helicopter pushing over the fuel tank doesn't make a sound until it is revealed by the camera. The Ganados are alarmed only by the creaking sounds of the tank. The helicopter makes so much noise that they should have heard it coming long before it arrived at the tank.

In the beginning of the second chapter, Leon kills one of the villagers and his body is left in a corner of the room with blood stains and his weapon lying next to him. However, once the player resumes control over Leon, the villager along with his weapon and bloodstains have completely disappeared.

Verdugo, Salazar's "right hand", pierces holes through the floors and ceilings with its tail, yet these holes don't stay there when the tail is retracted.

In one camp area within the village, where the players first encounter The Bella Sisters, it's necessary to defeat one of the sisters in order to get a key that unlocks a locked door that leads further into the camp which leads to the exit to the next area. However, if the player encounters the sisters with Ashley accompanying them, any villager that captures her can simply push the locked door open, enter it and immediately lead to Mission Failure.

During game play, Leon is dragged around the lake in a boat after a rope tied to the boat's bow gets wrapped around the tail of the lake monster. However, after the monster is defeated, the rope is suddenly no longer tied around the monster's tail, and gets wrapped around Leon's leg. If the rope is loose enough to untie itself from the bow of the boat, then Leon wouldn't have been dragged around in the first place.

If you're playing the GameCube version with alternative uniforms (Leon wears his classic Raccoon city Police Department uniform, Ashley wears white pants and a midriff-exposing top), during the knife fight between Leon and Krauser, Ada stops the fight, and Krauser says "Well, if it isn't the bitch in the red dress?" when Ada is wearing her Alternative suit, which looks nothing like a red dress.

In the cutscene where Leon, Ashley and Luis are in a cabin about to do major battle against an army of Ganado, Luis asks Ashley to introduce herself as if he doesn't know who she is. Earlier in the game, Leon shows Luis her picture and Luis was able to identify her. How is it that Luis was able to identify Ashley in a picture, but doesn't know who she is later on?

In a cutscene late in the game with Leon and Ada, Ada stabs Leon in the leg and falls down. A few seconds later, Leon stands up and moves around as if he didn't feel anything.

Beating the game unlocks different costumes for your character to wear throughout the game when playing again. However, sometimes the character's original clothes still show up.

If you shoot at the first Garrador in the prison before it breaks free, it takes no damage, but when he breaks free, you can damage it.

When playing the game in normal mode, the Merchant can be shot and killed, then later reappears with no ill-will towards Leon when meeting him at another location.

No matter what weapon you have in your hand before the cut scenes, Leon always has his initial handgun when he's seen in those scenes, whether you have the gun or not.

In the castle tower, there's a file that says that the Novistators took Ashley and to stop Leon the gears in the clock tower need to be jammed. The contents of the letter just happened a few minutes earlier. So there would of been no time to jam the gears and write the note and leave since the time Ashley was taken by the Novistators 2 minutes earlier.

After defeating the main village level in the beginning of the game, before the villagers are called away by the sound of the bell, if you destroy all the doors and windows in the vicinity before moving to the next level, the windows and doors are replaced when you travel back through there after rescuing Ashley.

Boxes broken in earlier stages are sometimes replaced and refilled when traveling back through those areas with no explanation.

Near the end of the game, Ada is shown belly flopping off a platform directly over a rising helicopter. Seconds later she's safely seated inside the rear passenger seat of the helicopter when her leap would have lead to her death.

Leon is shown continually throwing his combat knife during cut-scenes and accurately hitting his targets. The type of knife he's using is designed for close quarters combat. It would be impossible to throw a close-quarters combat knife because it's too heavy between the handle and blade, it's not aerodynamically shaped or evenly balanced between handle and blade and would never effectively hit any of his targets.

The jet ski at the end of the game isn't tied down to anything and yet the water current doesn't move it at all.

An enemy is shown firing a homing missile from an RPG-7. RPG-7s only fire unguided rockets.

During a cutscene, Leon enters a barn that leads to a boss battle against the village chief. The village chief was able to enter the doors silently behind Leon's back without him even noticing. In fact, the village chief entered the barn without even opening the doors.

Although walking into bear traps can hurt Leon and Ashley, the villagers can walk through bear traps unaffected.

NPC Limbo: After freeing the dog from the bear trap you see it vanish suddenly once it jumps the fence. Shortly after the villagers watching you on the ledge vanish also. This is not visible in the GameCube or PS2 versions, only on the PC version where it is presented in Full HD and with enhanced lighting. It is also noticeable on Wii, in the "Professional" mode.

In the chamber where you first meet Salazar in a cutscene, during gameplay, there's no one on the balcony, but then a shot later, when the cutscene starts, Salazar and two Verdugo appear.

Clipping: Leon's legs go through vertical objects to his right, including nearby fences and ledges he jumps over.

Leon and Ada constantly leap off of tall very high ledges during game play, but they never get hurt when they land regardless of the height.

A glitch exists where the player can double Leon speed by switching between a grenade and a weapon.

It is possible in some cases to reload checkpoints and bypass enemies.

Incorrect grammar in at least one of the cutscenes.

Weapons and equipment are carried in a briefcase, but Leon isn't visibly carrying a briefcase.

The cannon that Leon uses to destroy a gate at the castle entrance isn't pointed directly at the gate when it fires.

It is possible for the player to break through what appears to be reinforced glass, which is difficult to break.

When you reload the Mine Thrower, it, like the Rifle (Standard and Semi-Auto), finishes the sound on the Wii Remote about 2 seconds before Leon finishes reloading. Upgrading the Reload Speed once, will fix this.

The game is set in Spain, but the natives speak a Latin-American dialect of Spanish. The most obvious example to a native speaker is the use of "aca" instead of "aqui" which means "here."

When reloading the Rifle (standard; Wii Edition only), The speaker (on the Wii remote) finishes the sound but Leon isn't finished reloading. Upgrading the reload speed once, however, will fix this. This also happens with the Semi-Auto Rifle.

Leon and Ada continually get captured during the course of their scenarios, yet every time they get captured, their weapons and items stay on them. Who captures someone they intend to kill or torture but leaves that person's weapons on them in case they feel like breaking free and retaliating?

Any time Ashley gets recaptured by the villagers during game-play, it leads to Mission Failure. However, every time Ashley is recaptured during a cut scene, the mission to rescue her continues.

It is revealed through a memo written in English that Luis managed to create a machine within the Illuminados' laboratory that can kill the parasites in the host's body. However, the memo indicating this is immediately accessible to the Illuminados' soldiers whose higher commanders can speak fluent English, but by the time the player gets to the machine, it is fully operational.

During the second part of the game, Osmund Saddler stops Leon from taking Ashley and reveals his master plan to him. Doing so reminds Ashley that she has been infected with the same parasite the Illuminados have and they wish to take her away from Leon. However, the master plan is to have Ashley brought back to the United States infected; if the Illuminados had let Leon take her back without having reminded her or trying to stop him, then their plan would have worked, especially considering that they are the only people who have the cure, yet their own leader stops that plan from happening.

Leon is equipped with a very heavy duty knife. However, he never uses it to simply cut the rope when the boat is caught by the lake monster, and instead allows himself to be dragged around at a disadvantage until the battle is over.

Salazar knew about Leon before they even met. In fact, Salazar was able to jack Leon's communication radio without meeting him first.

Despite having numerous weapons at his disposal, such as a Hand Cannon, Chicago Typewriter, and a Rocket Launcher, Leon only uses a harpoon to defeat the lake monster, when he uses those available weapons for every other enemy.

Salazar calls Mr. Kennedy 'Mr. Scott'.

When the helicopter arrives, it doesn't shoot at enemies except in cutscenes.

If you don't go to the island's radio tower and radio for backup, later on, a helicopter, piloted by Mike, still shows up.