WRC3 has working FFB, voice chat, variety of cars to drive (not all in multiplayer) and the default suspension settings is stiffer than WRC4 making it easier to slide, rotate and roll the cars.
WRC4, sharper graphics, sound break up in single player mode, no FBB for road surfaces, no chat, less cars, removed New Zealand, added Australia, changed stages Germany, Finland, Sweden and Italy. Lots of cheaters with hacked mtx files taking all the fun out of competition
Both games work very well with wheels once you get it setup to your liking. First patch fixed some bugs and created new ones. Second patch removed the first patch,...it was that bad!
Still if you're looking for a dedicated rally game,..WRC4 is the one.