What square 4 plug are we talking about can't recall a spare plug. Where does it connect to?
I had this very problem with a new motherboard. The 4-pin plug in question wasn't seated properly and the motherboard wouldn't boot. It's usually 4 wires - often 2 black, 2 yellow - running from the PSU to the motherboard. Most I've seen connect near the CPU.
Edit : For your board, the connection circled in green.
