Looks like this plug is the reverse of a PCI-E power supply plug (polarity of pins P1,2,3 and P4,5,6 reversed from those of this power brick). Easy enougth to cut and splice the wires of course.
Or, new egg sells a 4 pin PSU Molex plug to 6 pin PCI_E plug converter for ~ $4 plus $6 s/h.
I also now understand that the round pins will fit into square holes - so the plugs should mate OK.
My remaining question is then concerning available watts - PCI-E is specified to only go up to 75 watts - but this may be conservative. Depends on the rails of the PSU I guess?