There's plenty of race fans that have put a ton of time into Forza Motorsport 3. These people will be glad to know that they'll be rewarded for that time and effort in Forza 4.
Turn 10 says that your profile from Forza 3 can be imported into Forza 4 through a feature called Profile Import. Based on player level and and the cars in your garage, Forza 4 will reward you with gift cars, bonus gifts and credits. This means that you'll pop in Forza 4 and see new rides in your garage.
There's also good news for those who spent a lot of time making layers and liveries in the Livery Editor: You will be able to import those layers in Forza 4. Nice. Thanks, Turn 10.