When posting a thread of troubleshooting nature, it's customary to include your full system's specs. Please list them like so:
I too out the hdmi cord on my main monitor to plug into a ps4
Did you remove the end at the monitor or the display output on your PC to tether to the PS4?
Power down the system, turn off the wall outlet, reconnect both monitors to the PC, power the system back up and see if you're able to get dual monitor functionality.