Might be a setting in the bios. Also bring up Task manager and click on performance tab, is it showing 4 cores?
If not, On Windows 7 click on start, and in the search box type Run, and click it. In the run window, type in "msconfig" and press enter. Click on the Boot tab and click on Advanced options, Make sure the number of cores is selected, and select 4 in the drop down menu and reboot your sytem and see if you got all 4 cores.