From what I can gather this is what you get if you were level 50 in Forza 3 and choose to import your profile.
Quote from forza forum "After importing my FM3 profile data I was awarded with all the cars for being a level 50(Abarth, Focus RS, Camaro SS, R8 5.2, BMW M3 GT2, Veyron, DBR9, and 908), the FM3 VIP cars(Corvette ZR1, 430 Scuderia, Superleggera), and the random cars from FM3(DeLorean, LF-A, RX-7 Spirit R, 458 Italia).
They even give you 60,000cr.! "
Looks like no matter how many CR you had the max moved over is 60,000
You receive a car if you have reached certain levels. The more levels you reached the more cars you will receive. If you have certain cars in your garage that you own, they will also transfer over to Forza 4.
Go here to read the official details from Forza and see a list of cars http://forzamotorsport.net/en-us/underthehood2/
Also found this info list of the cars you can choose as your free car after completeing each level. In Forza 3 you were just given a car. In Forza 4 you are now given a choice. One thing is the price of the car is not shown at the time of your choice so it would be in your favor to know what a car is worth.
At one level you are given the choice of a 1966 GT40 MKII or a 1967 Ferrari 330 P4. The price difference is several million but doesn't show at the time you have to choose. It also helps to know so you don't buy a car if you will eventually win one for free.
SPOILER- Here's a link to a complete list, level by level of the cars you will get to choose from and their value. DO NOT click if you want to be surprised as you level up. http://www.gamespot.com/features/6338910/index.html
One thing I haven't figured out yet is how to convert my paint designs to vinyl groups so I can import them to Forza 4.
Right now I'm out the door to pick up my LCE from Best Buy