Hello Bickel99
I have not found a function for this in the BIOS/UEFI either.
However, at that time I simply plugged in my graphics card and connected it. That worked immediately. In the device manager, only my GTX 980 is displayed.
If your graphics card has several connections, I would try them out and make sure that the monitor has selected the correct input.
Good luck
Yes, you can use all 4 RAM slots at the same time (why else would four slots be installed?).
You may not be able to use the 3600 MHz, depending on your CPU.