Yes there are people who look specifically for 4K , our analytics data shows that.
We have a few sites reviewed that have 4K. You can peek at their video specs too in the reviews.
https://x3guide.com/categories/4k-videos
For instance I think the Wank it Now 4K vids @ 16mbps look great. Of course it must depend on how much motion the videos have too, hardcore pounding might require higher bitrates.
I suppose you should just do some tests, have a few people judge. Shoot for the bitrate that looks good to everyone, or maybe a little bit higher just to be safe.