Would tue human eye notice any difference between 4k and 8k in VR?
Definitely. Current headsets have 110 degree fov. Normal tv viewing max 30 degree fov. So the difference between 4K and 8K per eye is comparable to 1K and 2K on a screen. Or 540p and 1080p. The human eye as 150 degree fov per eye. To get the clarity of 1080p on a screen (at recommended seating distance) you need 10K per eye. (Although with the optics putting higher pixel density towards the center, 8K should be sufficient)
As an upper limit, the human eye can tell the difference in clarity up to about 150 pixels per degree x 150 derees = 22.5K per eye! Of course the human eye can only see that kind of detail in a 3 degree cone in the center from where vision quickly degrades. Hence pupil tracking and foveated rendering will be the most important improvements going forward.