I just bought this monitor a couple weeks ago and was using it for my xbox one. It was working just fine. I tried to plug in my ps4 and all i get is a grey screen. I tried booting up into safe mode on ps4 to change the resolution but all i got was the grey screen. If i plug the ps4 into a tv and boot the system up and get to the home screen i can. Then i would switch the HDMI cords and it would come through the monitor on the home screen aswell. But when i tried to launch a game it would go directly back to the grey sceeen?
Are you using any adapters? And have you updated the PS4 recently? An update was issued for the PS4 not long ago addressing this specific issue.
Please try using a different hdmi cable also reset the PS4 settings and try to reconfigure the video settings on the PS4 if possible.