Actually, normal people don't rub their nose against their display but actually sit at a comfortable distance where you wouldn't notice a difference between 4K and 5K anyway - but still have more usable screen space.
I guess your UHD display (not 4k) has around 110 ppi, the iMac (which is actually more than 5k) has 218ppi. That is 2x as much. If you do not notice the difference from a comfortable distance, you have bad eyesight.