That PSU is garbage. It's not 650w by any means. It delivers 380w at best in the 12v rails which is what matters in modern PCs and it should have been labeled as 380w in today's standards.
The 4+4 from the PSU is meant for your CPU only. It's not meant to be put in the GPU and from what you said it appears that it only has a 6pin. You absolutely need to have every connector plugged in the GPU and adapters are an awful idea. They are fire hazards and can destroy other components. They are generally not acceptable.
That PSU was never meant to power such a GPU that's why it doesn't have the proper connectors. You need to buy a new proper PSU with all the appropriate connections.
From the following list pick anything that is tier B and...