its very impressive what is still being eeked out of the 360 considering its age. the difference between forza 3 and 4 is like the difference we see now between fifa 11 and fifa 12. no monumental huge changes apparent, but every single thing has been subtly an slightly improved, which combines to make a better game. i suspect this WILL be the last defining race game on the 360.
you should definitely try dirt 3 though, and if F1 is your thing you will find the game absolutely sublime. ive had pretty much every serious driving game thats been out since the original xbox - all the PGR's, colin mcrae's, forzas, 2 most recent F1 games, as well as tried the best of the rest but they never got me hooked. things like burnout, need for speed games, grid etc, not good enough if you've got PGR, forza and Dirt to play.