I'm trying to set up my Roku Express 4K and it is correctly detecting that my TV has 4K HDR, but it will not change in the settings to anything higher than 720p. When I click to change to the higher resolution, the device resets and when it comes back on there is an error message saying "you didn't confirm the new selection so the previous display type is restored" and it goes back to 720P.
After you complete Setup in 720p, from Roku Home Screen you can navigate to Settings...Display Type (press OK or use right arrow) and change the Display type there. After selecting the correct type, does a quick test and presents options 1. OK, go to "x" 2. Cancel 3. I've changed something, retry
Just highlight the appropriate action and press OK. (Screen should flicker and ask for confirmation)
In the event test fails, another option is presented "Force output to 4k UHD "x" Hz.
When you change the Display type to a new setting it runs you through several screens: