Resident Evil 4 was the first title in the series to implement a third-person view rather than the original fixed camera angles its predecessors had. Its mechanics have been completely revamped to incorporate fast-paced gunplay, quick controls and shoot-outs involving massive crowds of enemies in more open areas.
The saving system was also changed, there are no ink ribbons anymore however typewriters are still present. The game can be saved an infinite amount of times though there is a limit on how many different slots the player can save on. There is no chapter select either.
The game is littered with quick time events, even during cutscenes, to keep the player more engaged. In a few instances a QTE can appear during specific battles in real-time (i.e El Gigante, Verdugo, Krauser, Saddler etc.), certain QTE attacks can kill Leon in one hit if he fails the prompt. In the Wii version, some QTEs are based around the motion controls of the Wiimote, where the player needs to shake the controller or move in a circular way to turn cranks.
One key feature of Resident Evil 4 is Ashley, throughout most chapters in the game, Leon will be escorting Ashley through the map, the player can issue three commands to Ashley, either "wait", "follow" or "hide". The "hide" command can only be done within certain objects in the map. If Ashley is killed, the game will be over. The player can harm Ashley (which she will then be killed in one hit if a fatal weapon was used; eggs being one of the few exceptions) and she has a separate health bar than Leon. In some cases Ashley is needed in order to progress or open areas that Leon can't reach by himself and in other points of the story she will be taken away and the player will progress without her.
The inventory is also remarkably different from any Resident Evil title yet, dubbed "Attache Case", the new inventory now has a grid-like system where weapons and items have specific sizes which can be turned around in a "tetris-like" fashion to fit into the case. The player can upgrade the case a total of 3 times, each costing more pesetas (currency) than the other.
And, in order to upgrade the inventory's size, the player has to find the Merchant. The Merchant is another mechanic unique to Resident Evil 4, it is meant to be a "shop", but as a character. The Merchant has a pre-determined spawns at various points of the game and will not harm the player, though the player can harm and kill the Merchant. Even when killed, he will still spawn back once the player comes back to the spot where he is supposed to be. New weapons and upgrades will become available from the Merchant as the player progresses through the story, though there are a few spots where the player can find new weapons (such as the Shotgun or the Broken Butterfly), there is no way they can upgrade a weapon without the use of the Merchant.
Not only does the Merchant appear on the main game for Leon, but he will also appear in Separate Ways for Ada. Separate Ways is one of the minigames available in Resident Evil 4, which is available in all versions of the game but the original one on the GameCube, it is unlocked after beating the main game as Leon once, beating the game also unlocks other bonuses, such as The Mercenaries and Assignment: Ada, as well as the "Special 1" costumes for Leon and Ashley.
In terms of difficulty, Resident Evil 4 offers 3 or 4 difficulty modes, depending on the region and console. Overall the difficulties are: Amateur, Easy, Normal and Professional, with Amateur only being available on the Japanese version but not in the GameCube version. The biggest difference between difficulties is that on Amateur, Leon starts out with the Shotgun in his inventory and some parts of the game are unplayable which allows the player to skip them and move on to places they are not normally supposed to. On Professional, enemies do more damage and are more agressive, however, the dynamic difficulty is turned off in this mode. Dynamic difficulty automatically changes enemy's damage values, agressiveness and item spawns based on how the player is doing during their playtime. In some cases, the player is able to despawn enemies in later areas by tricking the system via taking deliberate hits.