I've had a little play with Forza 4 today, and from what I've seen I'm thoroughly impressed. I love the way your career is dictated by the track as opposed to the car / performance level. I also love the way that the AI actually push you in a race, it adds so much to the realism that all sim orientated racers strive for.
Playing with a wheel is pretty good fun too. My friend bought an R34 Skyline and tuned it to A600. At pretty much every track it grips like a snail to a wall, even with simulated steering the fun just keeps thumping through.
Any leagues that get set up should be thought about properly, to gain the most from the game as possible. The RD Forza 3 league is a good example of an all round racing experience, that encourages players to tune themselves with a wide variety of cars and upgrade restrictions to adhere to. If Forza 3 is anything to go by then were in for a good time with Forza 4.
