Since the 16-pin connector was introduced and later revised, melting adapters and PSU/GPU-side connectors on some of Nvidia's mainstream GeForce gaming GPUs have been rampant. Two of MSI's recently ...
I'm hardware-refreshing my old HTPC to an i5-3570K on a MSI Z77A-G41 which has an 8-pin 12V CPU power connector, and the PSU I have is an Antec Earthwatts EA-380 PSU, which has a 4-pin ATX 12V ...
The first one does the opposite of what you need; It turns a 4-pin PSU cable into an 8-pin one. I would go with the second one, that looks like what you need to do just fine.