Four beeps means there's a problem with the logic board, although I don't know which one it is. According with Apple manuals, it means "Bad checksum for the remainder of the boot ROM ". That doesn't mean a problem with the boot ROM, but a problem in the verification test each system runs in each boot. Verify you've placed the jumper block properly, that might cause the problem. Maybe one of the jumpers you were using for the overclock is broken. If you're using the original block of jumpers, you shouldn't have that problem.