Our Dish DVR shows that we can watch some streaming movies in 4K, but like the YouTube and Netflix, streaming is still not as good as a downloaded digital version or a media based option for 1080p HD, so in 4K it probably isn't that great on a monster TV without native content.
The streaming providers mainly have upscaled video, the same thing that a 4K TV can do, but upscale, even with great algorithms behind it, still isn't "true" 4K.
SO it is a mixed bag from 4K content, HOWEVER, 4K TVs tend to have better imaging technology in them for better colors, contrast, black levels, etc. So a "4K" TV should be the best option for watching 1080p HD, upscaled or not.
Some of the trade publications out there (in print or online) have said that for a little while now that the best quality TVs seem to be 4K TVs even if not watching 4K content.
