I have a board that allows cores to be locked or unlocked, so I would say it's motherboard-dependant. Of course, when I tried to unlock a core on my fx 4100 chip, the computer wouldn't boot, but my guess is that there just isn't a 5th core on that.
Anyway, yes, you should be able to disable them if you have the right board, and it shouldn't damage your processor. Also, can you not pump that baby to 4.3 ghz on stock cooling already?