I recommend you get a M2 card. The ultra m.2 slot on the pro4 should work and I have found them to detect easily without any issues so far. Just make sure you buy one that has good reviews. The first one I bought was from BeeEaster on Amazon and it was starting to burn up as soon as I turned on my computer. I did not have the power cable connected to the m2 card as is recommended.

Wanted to provide my experience with a few boards but for anyone looking to get a AM4 mining board I just wanted to say that AsRock Killer SLIac X370 seems to work right out of the box with at least 5 cards. I used 5 of the slots on the board and it booted every time I added one without a single hiccup. It has two additional M.2 slots that are both designated for PCI use so I believe it could do 7 as I have had good success with the M.2 expansion slots on another AsRock board.

These are the other ones I have tried:

Asus Prime B350 Plus - Worked with 4 GPUs with no problems and is stable but it only has 4 PCI slots and it seems like the M.2 would not detect at all for some reason on this board.

MSI Krait Gaming X370 - Big disappointment. Could not get it to detect more than 2 cards on the PCI slots.  Tried different BIOS versions and settings.

AsRock Fatal1ty Gaming K4 AB350 - Have it running with 4 PCI + 1 M.2 currently but to get that 4th card working I have to unplug the usb from the riser of the 4th PCI card when it shows the windows loading screen. It was a trick I learned here but I have had to to it every time my system restarts or else it just constantly reboots. I was gonna try the second M.2 slot also but saw in the manual it is only for storage unlike in the Killer X370 board.