Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
I can play a number of 4K files I've downloaded without an issue. I did run into just one that I had to run through Handbrake. See this site for lots of demo material, or this one. . I can play them on either my Ultra, my Sharp TV or my 4, although the HDR demos look pretty bad on the 4 since it's connected to a 1080 set without HDR support. But with the other two they look fantastic.
Dan Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.