20/20 vision is by definition average. I'm a little better than that, but when I say that the difference between 1080p and 4K is blatantly obvious, I have confidence that it will be just as obvious for the average person reading this.
In 5-10 years when we start talking about 8K (hopefully sooner than later), it will be the same discussion all over again, and I will tell you that the difference between 4K and 8K, at the same distance you use your 1080p TV right now, will be blatantly obvious (as long as the content you're looking at is actually good quality).
Beyond 8K things will get a little fuzzy, but by my own measurements I could potentially use a 30" desktop monitor (3 foot viewing distance) with as high as 16,000x8,000 resolution before individual pixels start blending in with the inherent signal noise in my eyes.
I think 8K would be a good resolution for industry to stick at, with maybe the odd 64 or 128 Megapixel screen made for special people like me.