SMPTE recommend a viewing angle of 30 degrees, which matches THX (26-36).
Assuming you have 20/20 vision, You won't be able to tell the distance between 4K and higher unless your screen fills more than about 40 degrees, in which case you are losing detail at the edges.
An 8K monitor on your desk may make sense -- if you're say 3' away from it and it's say 60", you'll start noticing a difference between 4k and 8k, however you will be focused on one area of the screen, rather than the entire screen.
Even with 4K, for most people watching television/films the main (only) benefits are HDR and increased temporal resolution (60p vs 60i/30p)
All the 8K stuff I've seen comes with 22.2 sound to help direct your vision to the area of the screen wanted. It certainly has applications in room sized screens where there are multiple events going on, and you can choose what to focus on (sport for example).
If you were to buy a 32" 8K screen - say the UP3218K, about 28" wide, to get the benefit of going above 4K you would need to be sat within about 30 inches. At 30" you would have the screen filling about 50 degrees of vision. Even an immersive film should only be 40 degrees.
