It would help to know which PSU. The initial response from
@montymint suggested you use two separate cables, and I would agree with this based on my previous experience using just one that split into two at the end and causing problems.
You will be drawing over 250w through the cables (+ ~60/70w from PCI slot) and each 8pin cable is rated for 150w (6pin rated to 75w) While you can probably get away with more than the rating it is best to avoid it and be in the clear. I would check the PSU as a 750w should have more than one GPU (PCIe) cable, and 750w should be sufficient.
Also, I don't think there is a 4pin PCIe connector as they tend to be 6 or 8 (split ones being 6+2). The only 4+4 I know is the CPU power cable that generally plugs into the top of the motherboard.