So if your eye can't tell the difference between 720p and 1080p on nearly all modern televisions, what's the need for 4K?
Excellent question. There isn't one. Not as far as TVs go, anyway. You'd need a 2,160p TV over 154 inches diagonal before you'd be able to see the pixels. On a 4K 50-inch TV, the pixels would be roughly 0.011 inch wide.
Where's the crossover where 1080p and 4K become noticeable? It's not exact because of all the above mentioned variables, but suffice it to say at 10 feet, it's somewhere well above 77 inches.