I have been playing CMR4 on my PS2 for quite a while now. I enjoy the game a lot. CMR4 and WRC3 in my opinion are both great games and each has their ups and downs. To give you improvements in CMR4 over CMR3...
-You can drive the championship in something other than Colin's car. (example: 4wd, FF, WRC, Group B championships).
-The service area's are back (like in CMR2) where you have to decide what to fix on the car in your allowed time.
-Car's stall if you damage them enough.
-You can choose between both Deric and Nicky as your co-driver.
-I really havn't done an in depth of the physics, but I like the drive
-You don't win new parts by getting 1st in the championship, after each rally they have a test you have to past to get the part.
-Customizable rallies
-You can have up to 4 people playing in a championship at once.
I'm probably going to get the PC version, due to the fact it has network/online play in the game. I just need to get a wheel to complete the experience. My modded x-box controller will do for now on my PC until I get it.