PS4 to IMac?

Target display mode is not supported on that model of iMac. For more information you can refer to the following article to find out which iMac models can be used as an external display.

Use your iMac as a display with Target Display Mode - Apple Support

No, your iMac is not an external display it is a computer and no there are no work- arounds. You question comes up daily and the answer is always the same. You may benefit by reading https://support.apple.com/en-us/HT204592