What I read is that you need to sit so near the screen to see all details of the 4K resolution. Their conclusion:
"The chart also shows that a 4k upgrade is not worth it if you are sitting more than 6' away and have a 50" TV. Your eyes won't be able to tell the difference. Ultra HD only makes sense if you want a really big screen and plan on sitting closer to it."
So for normal use and normal eyes there's no need to spend more money for 4K, because you won't notice the difference with good old HD.