If your motherboard has ACC or "Core unlocker" as some newer ones have it you can set it to all cores and it will attempt to unlock all the cores. Be aware though that your 4th core is not guaranteed to work, and may make your system buggy, so if you unlock it and you start getting blue screens or crashing lock it back up again.
Depends on the motherboard you have. You need one with a SB710 or higher southbridge if I remember correctly. In the BIOS set ACC to On, then set Unleashing Mode to on, press the save and exit button and SHA-BAM if there's anything to unlock you just did it. If the 4th core is unstable you can try raising the CPU voltage a hair or two to stabilize it otherwise it may be compromised. You will then have to go into the BIOS and turn the core off. Don't turn ACC off because you want the L3 cache you will have unlocked. Test it with a program like Prime95 and get HWMonitor to check out CPU temperatures and make sure they stay under 60C. That last bit is more for overclocking but you should have a peek at the temps anyway.