I have the same mobo, i had the same error on windows 10. 4x 1070ti connected to the mobo windows only could use 3 of them, I found a makeshift bios from asrock for 4g option. It did not fix it either. The only fix I could find is just ditching the windows all together and install linux. I have all GPUs working now. For testing I also added another card it can use 5x GPUs without problem.
Agree. There's a LOT to like about converting to a headless Linux build. Windows has its strengths, but to me, a simple miner that requires no license or automatic updates and restarts itself mining again every time it reboots is a joy.