Below is a list of notable and replicable glitches known to be in Resident Evil 4. Some glitches are only available in specific versions of the game.
The most notable RE4 glitch is the Ditman glitch, it allows the player to run & perform actions at an accelerated pace. It was first posted on Gamefaqs.com forums from a user of the name Ditman, thus the glitch was named after his username. The story was explained that Ditman and his brother discovered the glitch by accident, and figuring out what had happened, they had to share it.
In order for it to be executed, you need the Striker shotgun. To perform the glitch, you need to aim the Striker, enter the inventory before the laser sight appears, equip any other item other than the Striker, then exit the inventory. After resuming, Leon will begin to move and perform actions at 1.5x the normal rate including reloading, knifing, and more. This works in all versions of the game.
The glitch will be disabled if the player re-equips the Striker, climbs diagonal ladders, kicks down a ladder, opens a door, gets hit by any attack, uses any healing item, a cutscene is triggered, aiming a shotgun (including the Striker, which can be re-equipped after activating the glitch), or a loading screen is triggered (e.g. green text doors)
Other than making the game easier, an interesting application with the glitch is the ability to fall outside certain locations of the map, or cause Leon to float above or below the normal ground level. This is done by repeatedly using any movement prompts such as "Jump over", "Jump down", or "Climb" while the glitch is enabled. There's a chance Leon will end up standing in a different place than he would normally land. Places where this can be done include; going up or down the ladder in the village, jumping the fence in the farm, using the ladder in the Merchant's hideout, and many other areas.
Going outside the map can allow you to skip some chapters or cutscenes, and even "triggers" (e.g. enemy spawns). For instance, the fight with two El Gigantes can be skipped in Chapter 4-2 by using the zipline, then running back and using it again before the zipline resets itself. By doing this you'll go out of bounds and go straight to the next area by the green text door. You can not do this without the glitch as you are too slow to reach the zipline before it resets. Going back into the room, the El Gigantes will be standing behind the big door and remain passive. Damaging one enough to stun it will turn it aggressive, and jumping on its back will teleport Leon outside of the small room. The cutscene will play after the sequence ends, showing the El Gigante that was attacked getting back up. This also causes Leon to permanently hold the knife until exiting the area. Killing both without first teleporting back to the center makes travelling backwards impossible.
The player can also skip the entire Salazar boss battle by standing on a very specific spot and jumping down with the glitch enabled, Leon will be walking on the air and he's able to reach the door and walk through Salazar. If the player comes back to the room he'll be able to get the money Salazar would drop as if he was defeated, except he's still there. Every time you go back to the room Salazar will say "I want you to suffer like Ashley" and his eyeball will now be completely white. If you stand where Salazar is sometimes some QTEs will show up on-screen and you can do the dodging animations, but even if you don't Leon can't get hit.
Going outside the map also reveals peculiar items laying around. In the Farm area, grenades, herbs, first aid sprays and a shotgun can be found. In the Merchant's hideout in Chapter 2-1, the player can find ammo, health items, and a shotgun (which, when the screen to pick up (yes/no) appears, the shotgun is shown as the green shotgun ammo case, used in almost all other previous Resident Evils to indicate Shotgun ammo). When picked up, the shotgun will turn into Ada's shotgun and if equipped it can cause the game to crash. Also floating outside the map in this area is part of a wooden gate, which floats above Leon. There are numerous other oddities outside the map's boundaries, including doors on the opposite side of a wall that has no door in-game, and so on.
While the glitch is active, reloading with the Red9 is faster. If one interrupts the reloading animation by moving, the ammo clip that Leon would normally push into the gun will remain there, giving a humorous effect. This will only last until the next reload, however. The same occurs for the Broken Butterfly - if the player reloads while the glitch is active and allows the animation to complete, Leon will reload quicker as normal, but the chamber shall still remain open, only to snap shut by itself immediately as soon as the normal reload time has allotted.
When the Ditman Glitch is activated while holding some weapons, the quick turn command (down+run) is malfunctioned. Instead of performing a quick turn, Leon turn around then quickly turn back to the original direction he was facing. This can be solved by moving forward or aiming immediately after the quick turn is finished.
This glitch only works in the Gamecube version, with the TMP and its stock equipped, the player can shoot breakable locks from the other side of the door, with this, the player can skip the entire Garden stage, as well as get the Rocket Launcher in the Gallery room before triggering the bridge. It's also possible to skip Ashley's trap in the Weapon exhibition room and get the Queen's Grail. If the player triggers the Salazar cutscene in that room, he will be seen walking through a door.
By going in and out of the crank animation, there is are certain frames where Leon is completely invincible to any sort of attack. This can be very useful in the Castle entrance stage, if done right Leon won't get hit by the incoming flaming boulders, nor any attack a Zealot does. The downside is that Ashley is still vunerable, but thankfully as soon as you shoot the cannon, all enemies despawn.
Frozen zealot glitch
In the Castle parlour, before jumping down to where the exit door is located, if the player throws an Incendiary Grenade at the Leader Zealot and jumps down immediately, a cutscene will trigger and instead of showing the zealot running to a room, it'll show him getting burned. If the player skips that cutscene and runs up to where the leader zealot is, he will be laying on the ground and will stand up. However, instead of running away likes he's supposed to, the zealot will be standing still and won't attack the player.
Prison Regenerador glitch
Once inside the area, do not fire any weapons. Walk (do not run) down the hall along the left side of the wall. When you get to the second cell turn left and the Regenerador will be right there. It will not attack unless the player steps too far away from it, shoots it, or runs. Players will also notice its eye seem to be glitching and are rapidly moving in all directions.
Rapid dodge glitch
In the HD editions, during the first half of the fight with Bitores Mendez when the "Duck" dodge prompt shows just as he motions for a horizontal swipe, pressing the buttons normally makes Leon crouch once. Pressing the buttons rapidly will cause Leon to continuously redo the dodge until the player stops pressing the buttons or the prompt disappears. This glitch is more than likely due to the script file messing up for that specific part in the game.
Shooting Range ammo glitch
The shooting range ammo glitch, (or "Sparkler") is a glitch that occurs only in the NTSC Gamecube version of Resident Evil 4. After a player is done with the shooting range minigame, they should head to the merchant and shoot at him at point-blank range. It should be noticed that the bullets will go harmlessly through him. After, the player must "return" the weapons to the merchant, however, in several locations, the ammo the player has in their current equipped weapon, will be empty. (The health bar will display nothing at all.) The player won't have the ability to reload that weapon; In order to end this glitch, they just have to change the weapon to another or get out the room. Hence, their weapon is now infinitely loaded.