Thanks for your reply.

I have recently changed my motherboard since it died (ASROCK H81 PRO BTC) to ASUS EX-B250-V7 with 4GB of RAM. The newest BIOS is installed on the Asus and mining mode is enabled, and I am using it with 3 Vega 56's and one Vega 64. mmpOS only detects 2 or 3 of the cards, and if I keep switching them around it would sometimes detect the 4th one but when it starts mining it would crash in 15-30 minutes. I know this a long shot, but do you have any thoughts on things to try? I will probably install Windows and test with it soon.

More info:
-The card that is not detected is not always the same card, so it is not a problem with the cards themselves or the risers.
-A lot of people online complain that the red PCI-E port on the board causes problem, so I tried without connecting a card to it, but same problem.
-If I mine with the 3 cards mmpOS doesn't crash.