4000 PPI is not a comparable metric to the Pro Display XDR in this case because it's referring to the physical specs of the OLED panel. But the panel is almost directly against your eye and not at a normal monitor viewing distance.
A more relevant user focused metric would be "arc minutes" or "degrees" per pixel. This is not a commonly quoted metric.
As a rough proxy for pixels you would need to simulate a Pro Display XDR in VR, you could take note of how many degrees of FOV your monitor occupies in front of you at a normal viewing distance. It's probably between 30-45 degrees. Then note than FOV of a VR device would be around 110 degrees ... so 110/40 = 2.75. Then 2.75 ^2 = 7.56. So 7.56 times the pixels of an XDR to display the same device in VR.
Then note to simulate the "pixel" grid perfection of a retina display on another display that isn't aligned perfectly, you need something like 4x the pixels again to get rid of most aliasing effects.
The "degrees per pixel" equivalence also falls off if you like moving your head forward to squint at pixels ... so do you want to account for that worst case in your specs?
Then on top of that we want all the low persistence and low latency and high refresh rates that makes VR comfortable ... and we want basic specs like brightness and contrast and gamut to be as good as we've gotten accustomed to.
There's huge room for improvement when it comes to VR displays. It will be interesting to see how far Apple will move the bar forward, but it's hard to believe they will be anywhere close to perfection.
