4K resolution is 3840 × 2160 (12.6 megapixels)
8k resolution is 7680 × 4320 (33.2 megapixels)
Here is the problem, if you have 2 TV's, both are 1080P, but one is 50" and the other is 70", the 70" one will look like crap because of how big the pixels are as you have the same number of pixels you are just stretching them across a larger surface. To compensate for that you need a higher resolution (more pixels) for larger screens to look as crisp.
The opposite is not true though, if you have two 50" TV's and one is 1080P and the other is 4K, they will look almost identical. 4K is for screens 65" and bigger, 8K is for screen 85" and bigger. If
you look at most of the 8K TV's that are out, most are 85"-110" which is where that resolution will make a difference.