Resident Evil 4 Unlockables

After finishing the game you will unlock the Assignment Ada and Mercenaries-mini-games; the Professional difficulty setting; the Leon's Raccoon City Police Department costume, Ashley's pop star costume, and Ada's black costume; and the Mathilda handgun and Infinite Rocket Launcher for purchase (for 1 million Pesetas).

Get a four star rank in the following areas to unlock the corresponding character in the Mercenaries mini-game.
Ada Wong: Pueblo
Albert Wesker: Waterworld
Hunk: Island Commando
Jack Krauser: Castle

Get a five star rank with all mercenaries to unlock the Handcannon weapon. To get unlimited ammunition for it, fully upgrade all of the Handcannon's options.

Finish the Assignment Ada mini-game to unlock the Chicago Typewriter (Tommy gun with unlimited ammunition) for purchase (for 1 million Pesetas).

To avoid getting hurt by traps such as explosives and bear traps, use your laser sight and shoot at the explosive or the middle of the trap.

To find items, smash all of the crates with either your knife or a gun.

Wait until you are low on ammunition before upgrading a gun. The replacement gun will always have a full clip, giving you extra bullets that otherwise may be difficult to find.

Get a five star rank with all mercenaries to unlock the Handcannon weapon. To get unlimited ammunition for it, fully upgrade all of the Handcannon's options.

after you get ashley when you are on the mission where you have to go on the ski jump go into the cave nearby and kill all the monsters there, afterwards go on the ladder and the jump down (but do not choose to catch ashley) instead use a rifle (preferably a semi auto rifle) and take a couple of steps back, then use the scope to look under ashleys skirt you will see her panties and she wont grab her skirt or call you a perv.

Clear Leon's story on professional difficulty to unlock the PRL-412.