Almost identical to the 3rd Forza. Many of the same tracks and cars. Gameplay is still very fun and lengthy, however I feel it was shorter than the 3rd. I didnt get too active in the online mode so I can't say too much about that other than there's online "groups" you can make with your friends and race other "groups" similar to a gang or something you could say. The game was very fun, definitely a good rent if you like realistic driving/racing. The cars were in incredible detail with a new feature to fully explore each car which is kinda cool. Rent it, you'll like it!
If you played Forza 3, you're probably not going to notice too much of a difference, but the game (as a stand alone) is still magnificent. The game looks gorgeous, has a huge collection of cars to customize, and beautiful tracks (same as in Forza 3). If you're a beginner you can allow the cars to brake for you, or you can turn off ABS and all assists and try to stay on the road. It's got everything for a car/racing lover, including the Top Gear UK test track. POWER!!!