Under Boot - Advanced Options where it says "2 Cores", uncheck that box. There's a bug with that screen that once you check the box and select the Number of Processors it will never give you an option to select more processors. So once you select 2, you cannot select 3 or 4. Just uncheck the box.
Once the "Number of processors" checkbox is unchecked, reboot, the computer will then run with the default number of processors - 4 for you.