I remember when I built my first P4 machine that needed the extra 4-pin power connector. I forgot it, because all the ones I'd built before were standard 20-pin. I spent ages wondering why the fans were going round but there were no beeps and nothing on the screen. I ended up changing the CPU, RAM and then finally worked it out and changed the PSU for a P4 one with the extra 4-pin connector - and surprise surprise it all started up fine. :-P
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