Just like Forza 3, Forza 4 has a save corruption problem. This can happen even if you never move your save from your console, or even before you power off the console after the first time you play. How this happens, I have no fucking clue. It's complete and utter bull shit. But it's a thing, and I'm warning you.
Before getting too far into career, I recommend that you attempt to do EVERYTHING in the game once. Make a vinyl, apply said vinyl to a car, make a tune, post a lap time in Rivals, EVERYTHING. Otherwise, you're going to get 20+ hours into career, decide it's time to try your hand at making a vinyl, and you won't be able to because your save is corrupt.
No word from Turn 10 yet on this problem.
Looks like the game makes a dummy save that's corrupted. The one I had was only 40KB and I deleted it. Fixed my problem.