Machine will not power on with DP ports used

Hi, I have had a few machines that will not power on properly when 4k monitors are plugged in. The power light flashes orange, and nothing else happens.

The user has to unplug his monitors, boot the machine, then plug the monitors in again. 

I've had this problem on 2 machines, and both of them had 4k screens plugged in. 

The machines in question are Optiplex 9020M and a 7010SFF.

The 9020M has a single 4k screen plugged in, and the 7010 has dual.

Is there some kind of compatibility issue with the onboard graphics and 4k monitors?

Thanks,
Nathan