I like Forza 4 a lot and am still playing it but it looks a bit worse than 3 to me graphically, I think targeting 60 fps on the One X hurt it a bit with the draw distances. Of course this one will have a custom next gen version so it'll be a different story here.
Also if youre a big Forza 4 fan on Xbox, probably wait for a sale as it will almost definitely be delisted in a year or two. I regret not picking up Forza 3, Ill probably buy a 3rd party key on PC since those still work.