If you one of those people who hunt down every achievement in your games, and can't relax until you've secured them all, then you're going to have to pick up Kinect before you get Forza Motorsport 4. One achievement in the game is called "Look ma, no controller" and tasks you with racing down one track in Free Play using Kinect. You're also going to need a save file from Forza Motorsport 3 if you want the achievement "Forza Faithful".
You can find all the achievements in Forza Motorsport 4 over at Xbox360achievements.org.