It's more like 1080p monitor. The virtual monitor only covers a small part of the VisionPro's display. You can compensate a bit for a lack of resolution by making the virtual screen bigger or by leaning in, but none of that gives you a 4k display.

To really take proper advantage of the VR environment you really need the ability to pull out apps into their own windows, as than you can move lesser used apps into your peripheral vision and leave only the important stuff right in front of you. You also miss out on the verticality that VR offers when you are stuck with a virtual 16:9 screen.