I have a roku ultra LT from wal mart and an older one 5yrs old from best buy.
When streaming 4k they will not allow playback of anything but the 16:9 with black bars on top and bottom why? We wont get into dolby vision yet, another topic.
Are you saying it forces everything into 16:9 or you would like to stretch and distort the picture so that it fills the screen? That's a function of the TV, not the Roku.