After reading up on 4K and 8K, I might just skip 4K all together and wait a few years for 8K. Unless my current TV breaks, or 4K TVs drop in price a lot, I will hold on to my 1080p for a while.
I think the first 8K TV is huge and cost over $100k, but maybe by the time ATV6 comes out, they will be where 4K TVs are now.
The thing you need to keep in mind is that TVs generally cost more due to size than due to the number of pixels. In the same manner as silicon chips made from a wafer (which have a fixed cost), the more you can fit onto one wafer the cheaper it is to produce. Also, since a single imperfection can ruin the whole thing, the larger an object, the more wastage can occur and the higher the chance.
Thus, big TVs like that 8K one that was unveiled not long ago cost a lot of money because of a combination of early technology, aforementioned large display factor and yield concerns, as well as lack of economy of scale and probably just because they can charge that much because it's a niche product.
I bet you can buy the other recently announced 13" (?) 8K OEM panel in the low hundreds, if that. Although, I'm purely speculating.