Just bought a Roku Ultra 4K (model 4850CA) this weekend and am having nothing but issues getting this box to boot up. Right out of the box, using the provided power supply and HDMI the unit shows the Roku logo as expected but immediately freezes; sometimes the log will bounce and move but it freezes soon afterward.
I have attempted multiple factory resets to varying success, I have only once gotten to the setup screen before the unit freezes and have not gotten as far as a network screen to force an update. I have attempted leaving it unplugged for a couple hours with no change, I have booted it without the HDMI attached and left it for an hour, it still seems to just loop and never gets as far as the initial configuration window.
The instructions for recovery mode don't seem to force the unit into any other mode, instructions on this site only cover Roku TV manual updates/recovery situations.