Well, I thought I had seen at least one in the channel store, but I just went to look and I couldn't find any. However, I somehow have a screensaver titled "Nature 4K UHD Screensaver" installed, and it's definitely outputting 4K, which is probably why I thought such a creature existed. I wonder if this is some built-in Roku screensaver with special capabilities that you can't get with the normal API?
Me neither, i did not find others.
The "Nature 4K UHD Screensaver" screensaver comes pre-installed and is not removable. I bet it's RokuCo's own, in which case all bets what it can or cannot do are off, since they can do it outside the SDK.
Btw, same SS works also in 720p screen resolution... i imagine images get downscaled