All depends on the viewing distance. It can probably simulate a 4K display with you face very close to it. Or a 1000m wide 4K display 100m away from you. But in

All depends on the viewing distance. It can probably simulate a 4K display with you face very close to it. Or a 1000m wide 4K display 100m away from you. But in both cases you'll have to look around a lot to see the whole thing. A realistic case for a monitor would be a viewing distance about equal to the width of the screen, so taking up only about 50 degrees of horizontal FOV, with the whole screen in your field of view. There's no way it'll be able to render every pixel of a 4K virtual screen in those conditions. I'd be surprised if it looks as good as a 1080p screen, when you consider the resampling from the pixel grid of the virtual screen to the final output on the headset screen. (I guess you could argue that rendering 2 views gives an element of supersampling, though?)