Resident Evil 4 was, at first, developed and designed exclusively for the Nintendo GameCube. Late in the development cycle, Capcom announced that Resident Evil 4 would make an appearance on the PlayStation 2 as a port (with a few extras).
Because the PlayStation 2 version had quite a bit of extra time for development, a number of special features and changes were made to the game. Most of these changes are not significant and do not affect the overall plot or story of Resident Evil 4. You'll find very little in the way of main story differences between the Nintendo GameCube and PlayStation 2 versions.
The GameCube version of the game underwent further change before it's release in Europe. The German version had the most drastic of changes applied to it, with much of the game's content being censored and/or cut.
In 2007, a port of the game was released on the Wii, under the title Resident Evil 4: Wii Edition. This version includes all the bonus content from the PlayStation 2 version, as well as a new control scheme using the Wii Remote and Nunchuk. It's also compatible with the Classic Controller and GameCube Controller, for those who prefer more traditional control schemes.