Are you sure it's not the same as it was with Forza 3 to Forza 4? I saw that loyalty bonus thing, but I swear I've seen a few sources say you'll be gifted a few cars based on what your FM4 save says you have in your garage.
It's about 5 cars that you automatically get if you had them before. The list is online in various places, you also get loyalty bonus cars just for having played the last games-about 5 or so