Extra power source availability
Extra power source availability
Connecting the four-pin adapter to the CPU causes the PC to fail to start. The manual indicates it's intended for additional GPUs, but you have two. Your specs show an Intel Core i7-6950X, Nvidia GTX 1080 (two), SSD, RAM, and power supply.
Did you link a PCIe power cable to the CPU power? We can obtain an image of your setup, PSU, or motherboard if you'd like.
I also connected it, but I wondered if I should power the 4-pin connector there since there wasn’t a matching cable with the power adapter, so I brought in an extra one.
Without the additional four-pin connector, there are no major problems. The existing eight-pin connection provides ample power, so everything should work smoothly. Even if the PC runs without the extra pin, you can safely skip it.
Typically this provides extra strength for the CPU during overclocking, even though it's not strictly necessary. Running the system without it is acceptable. It's just an option.