Can Duet be used as a 3rd or 4th display?

Yes.

Duet will be recognized as an additional display by your Mac and PC and you can use Duet as your 3rd, 4th or even 5th extra monitor. The majority of modern computers can support several monitors, but if you are running super high res monitors (e.g. 4K) please make sure your graphics card can support additional display output before connecting Duet. Besides that, don't forget to arrange your displays as needed – here's how.