well isnt it all drawing power form the same place anyways? and that 4 pin is the normal 4pic cpu connector, it just makes it a 8 pin.
Check the pin-out of the connector... You are correct, in that the power all comes from the same place, problem is the difference between your molex and an 8 pin molex are quite different. Gkay's solution is a lot cheaper than buying a new PSU, and should work.
Personally, I'd get a new PSU. Antec makes some good units in the 650W range, as does Corsair, either would be a good unit at a fairly reasonable cost.