OP PM'd that his system is up and running now despite the crappy documentation on these
things.
He confirmed that to rig a single XXXX-in-1 for 4-player operation:
* The computer board is powered by an ATX power supply 20 pin.
* The main JAMMA board connector (P1/P2) gets wired exactly like a normal JAMMA harness with +5v/12v/Gnd from an arcade power supply.
* The P3/P4 daughterboard connects to a JAMMA harness (P3/P4 controls wired same as P1/P2 controls)
and with +5v/12v/Gnd power from the arcade power supply. (One supply powers both the main JAMMA board and the daughterboard)
Connecting power to the P3/P4 JAMMA isn't in the documentation, but is mentioned in one vendor's video.
Sorry, but no link to the video because of Rule 5. (The account is named after the vendor)
Scott