We don't see in 2k or 4k resolutions. The details our eyes can resolve depends upon angular resolution, which is about one arc minute. When it comes to TVs and displays, the maximum resolution we benefit from depends more the effective distance we are from that screen. Here's a diagram that shows the relationship between screen size, distance, and effective resolution:prove otherwise. with science please
How does this apply to VR? Simple. Because the "distance" of the display in current generation VR headsets is "infinity," we simply need to know the pixel density over its field of view. Anything over sixty pixels per degree is effectively wasted, due to the pixels starting to blur together. My Vive, for example, has an effective field of view of about 110 degrees. That means that it can use a display 6600 pixels wide, or somewhere between 4k and 8k. Keep in mind that each eye's field of view overlaps quite a bit, so the equivalent of an 8k monitor would be ideal.
The reason why I'm personally skeptical about 8k VR headsets isn't that our eyes can't take advantage of the pixel density. It's because of the huge graphic processing requirement for screens that large, while the fovea (which is where the light sensing cells are densest) is so small, with a field of view of only 5 degrees. Until foveated rendering and eye tracking become the norm, this size screen won't really be viable for widespread use.