I ordered and built my EVGA 650w GQ power supply. I have built and checked everything in my pc and everything turns on/works except when I use the cable labeled 'CPU'. I can tell it's not meant to fit with my motherboard (ROG Strictly B360-G Gaming), I'm guessing they're not compatible because when I force it in (not too hard that anything is breaking) and turn the pc on everything turns on for less than a second then turns off. However, the YouTuber who I got my build off made it work perfectly with the same parts, however he unfortunately time lapsed the build.My friend recommended me to get an adapter which I have purchased
I was wondering if this would work given that my motherboard has 8 pins and this one has 4, I searched it up and there's mixed reviews, some people say it works fine, others say it will slow your pc down, others say this is dangerous. I was wondering if you guys could recommend what I should do, thank you for your time.

Remember that one end is for the MB the other is the PSU
You Need No Adapter..........DO NOT USE ONE
Your CPU EPS connector is 2 4-Pin Connectors put together to create an 8-Pin EPS Connector. 

Second cable down on the left side is the correct cable you need. Your PSU comes with 2 EPS CPU Cables.

 
Put the two 4-Pin connector together with the Clip or Lock on the same side next to each other and plug into your 8-Pin EPS CPU Connector on your Motherboard.

Matthew.stockings
The split ends go into my mb, single one goes to PSU and pc turns on and off, when the cable for CPU is unplugged everything turns on, is this a different problem from a different part I've fitted wrong?

Maybe, need more info.
Your 24-Pin Cable is plugged in I take it.
Do not use any cable that has VGA on it and use on the EPC CPU connector.
Make sure you also plugging in the EPS CPU Cable in the CPU 1 or CPU 2 on your PSU.
 
"pc turns on and off" Check that you have no Motherboard Standoffs Sorting your MB.
The Hole count should be the same as the number on the MB Tray in your Computer Case. I count 8 Standoffs are used.
Remove you MB from the Case and retest. MAKE Sure you have a Cooler Installed on the CPU.
 

Time to bench test and count them Standoffs.

You are Very Welcome.
Keep us posted