I have a Corsair HX 750, but I lost one of my 4 by 4 molex cables. Planning on putting in another GPU but don't have the cables. I think it would be 6 pin to 4pin (preferably with multiple male molex)
Why would you be looking for a "4 by 4 molex cable"?
You should be using the four (6+2)-pin PCI Express Supplementary Power cables that are specifically used for graphics cards. They are the ones with the blue 8-pin connector on one end of the cable that plugs into the blue 8-pin modular sockets on the HX750 power supply unit.
If you've lost the extra (6+2)-pin PCI Express Supplementary Power cables then contact Corsair.