Turn 10 are rewarding the Forza faithful with the ability to import or transfer cars that you own in Forza 3 to Forza 4, as well as rewarding your driver level with cars.
Every driver level in you have below in Forza 3 will reward you with the corresponding cars in Forza 4:
Forza 3 Driver Level 1 = Forza 4 will import: 2010 Abarth 500 Esseesse
Level 5 - 2010 Ford Focus RS
Level 10 - 2010 Chevy Camaro SS
Level 15 - 2010 Audi R8 5.2 FSI Quattro
Level 20 - 2009 Bugatti Veyron 16.4
Level 30 - #92 BMW Rahal Letterman Racing M3 GT2
Level 40 - #007 Aston Martin Racing DBR9
Level 50 - #9 Peugeot Sport Total 908
So, if you are level 50 in Forza 3, you will get 8 cars for free when you start Forza 4.
Also, if you have the following cars in Forza 3, you will also get them in Forza 4:
2010 Lexus LF-A
2010 Ferrari 458 Italia
2006 Subaru Impreza S204
2002 Mazda RX-7 Spirit R Type-A
1969 Chevy Camaro SS Coupe
2009 Chevrolet Corvette ZR1
2007 Ferrari 430 Scuderia
2007 Lamborghini Gallardo Superleggera
1982 DeLorean DMC-12
These cars come from:
Community Choice Classics Pack:
1982 DeLorean DMC-12
Hot Holidays Car Pack
2010 Ferrari 458 Italia
Stig Garage Pack, which was part of the Forza Motorsport 3 Ultimate Collection:
2010 Lexus LF-A
Forza 3 VIP Car Pack:
2009 Chevrolet Corvette ZR1
2007 Ferrari 430 Scuderia
2007 Lamborghini Gallardo Superleggera
The remaining 3 are Forza 3 Unicorn cars:
2006 Subaru Impreza S204
2002 Mazda RX-7 Spirit R Type-A
1969 Chevy Camaro SS Coupe
These 3 FM3 unicorns will also be unicorns in FM4, as well as the following cars:
2007 Ferrari 430 Scuderia
2007 Lamborghini Gallardo Superleggera
So, if you have Forza 3, be sure to get your driver level as high as possible, and make sure you own all of the above transferable cars in Forza 3 if you have access to them.