Thank you VERY much for confirming 4-pin will work. It's kind of frustrating that the mobo manual wouldn't provide this sort of useful info.Absolutely sure that you just need the 4-pins (or the two systems I have running are an anomaly).
I have them in SM CSE-e300 chassis and the connector was included with the chassis. Note that newer e300 chassis are delivered with an 8-pin connector. Unfortunately I don’t have a link to the cable.
As for the P4 to DC cable, the only one I've found so far is here.
Any advice on power bricks? The SM branded ones seem very pricey but it also sounds like some of the cheaper ones are downright dangerous for the loads they are powering.