i had a similar issue so plugged monitor into one of the gpus connected via the risers not mobo. weird thing was a few worked some didn't, never worked out as one displayed monitor.
Yeah i tried this as well and the monitor didn't work either? I didn't try all of them but the one i did try i didn't have a display. Ill try the other 3 maybe tonight and see if that makes a difference. I mean if i can get ANY of them to work so i can see whats going on and start mining that's fine with me. As long as all 4 cards are recognized by the system...lol