SOLVED
Hi all, I am simply trying to connect my Raspberry Pi 4 to the PC (ASUS) and use it as a dual monitor. one for the PC and one for the PI. I have connected the HDMI from pi to the HDMI port on PC and nothing happens
Forgive me if this should be a question for the pi community?

I havnt solved it as such, but I have got a workaround. I simply got PI to login as Pi user automatically and I then just connect via VNC server and control everything through PC, thus eliminating need for a seperate screen,mouse and keyboard. That will work for me for the time being

Without knowing anything about your Asus PC I would guess its HDMI port is 'video out' only.

Yes MichealK,
I suspect you may be right. Back to the drawing board, or buy a monitor dedicated to the PI

Connect the Pi to your LAN and run a VNC server on it.
On your PC run a VNC client.

This would allow you to control your Pi headlessly in a GUI mode from a window on your PC.
If CLI is sufficient, then SSH would be easier to set up.

This would work for any pair of computers...