I mean that I am not very sure the human eye can even distinguish between 4K and 8K, provided at least 60FPS.

IMO we're hitting maximum levels beyond which there's no sense to continue -- except to sell us the latest and the shiniest.

Required resolution depends on field of view, which tends to be well under 40 degrees required for "movie" style experience for most people even now

However 4K isn't worth it until you get over 30 degrees, and any higher isnt worth it unless your tv is too large, at least with modern cinematic techniques.