Are you saying that the power connection on your graphics card is a 4-pin? Or is there a 4-pin connection on the mainboard?
If the 4-pin connection is on the graphics card, then I don't know. I don't recall ever seeing one, other than a standard Molex.
If the connection is on the mainboard, the power supply should have a specific 4+4 connection for that (it's not a PCIe connection). Just use one half of the 4+4 connection.