The titles with good lighting and vibrant colors are the best. Anything with dark scenes or otherwise dimmed or with poor lighting are not worth it in my opinion. Maybe if they were 60fps in addition but they are not, they are only 4K in frame size/bitrate.
I'm less worried about the individual picture quality and more worried about the bitrate being too low and that causing those almost VCR like 'lines' on parts of the video as it plays on higher resolution screens. I can't think of a way to show off the effect but essentially think about point of view videos where the action is close up. When the girl is riding cowgirl or reverse cowgirl and bouncing up and down going full steam, on a 4K monitor you can sort of see these lines forming on the girls boobs or butt (whichever way she is facing) as she bounces up and down. I also want to stress this is not screen tearing. These are localized lines just on the boobs or butt as they bounce up and down rapidly.
It's not end of the world stuff but I do notice it. I wouldn't notice this in a bluray for example. If I grab a Minori Hatsune bluray I have, her boobs bounce fine and the skin doesn't have lines on it as they bounce up and down rapidly. A bluray has an excessive bitrate of anywhere from 15,000 to 30,000 on a typical 1080p release.
I understand it's not practical to offer bluray like bitrates online... the file sizes would be massive. But I would imagine there is some sweet-spot slightly higher than 6,000 that would fix the minor VCR-like lines issue I see on my 4K monitor for 1080p videos. Say 8,000 or 9,000 bitrate.
Which brings me back around to my original question/issue of wondering what the 4K bitrate will be. 4 times the pixels is a significant jump and when you go from the raw 4K file and re-encode to reduce file size (and bitrate), will the video quality be good? Will I see the same lines if the movement gets too fast and the bitrate can't really keep up? Will it be better, worse or the same?