I got an AMD FX(tm)4100 Quad-Core recently and i noticed that only 2 cores are running.
I made some research but i didn't found any way to solve it. I rebooted my computer may times after checking or unchecking the checkbox for Number of processors, it still running with 2 cores.
The quality of the screen shots is not the best but I hope they will help you.
If you have any idea to solve this problem, I'll be happy to read them.