Forza 4 is pretty sensitive went it comes to running with other things in the background. If you're crashing/CTD during the loading part (end of titles when it says Microsoft Studios) make sure nothing is running that you do not need, check with task manager. I also discovered that changing resolution or even refresh rate makes the game crash/CTD during loading. So try changing resolution or refresh rate, my monitor runs at 100Hz but I found when I switched it to 60Hz (for other apps/games) then tried to run Forza, it would not always work, and vice versa.
My 2cs.
Ta, had tried closing tray tasks but will look into Processes too.