Your hardware is probably able to run in hyperthreaded mode (which is also activated in the kernel).
This options makes it possible to have virtual cpu's. In your case (and mine
), you have 2 real cpu's and 2 virtual cpu's. All 4 cpu's are used.
Not all all 4 cpu's run at the max speed, but roughly half the speed (a bit faster).
Hope this clears things up a bit.