Android TV UI is in 1080p. We’ll have to wait until Android 12 for it to be rendered in 4K.
Had the same issue my FHD TV being scaled to 720p. Found a solution here: https://www.reddit.com/r/AndroidTV/comments/k74adv/how_can_i_set_the_resolution_on_my_android_tv/?utm_source=share&utm_medium=mweb
What TV?
HDMI set to Enhanced?
Also, are you going through a receiver? Some receivers (like NAD's) report incorrect maximum resolution.
Because Aida is showing current resolution, and android tv UI is rendered at 1920x1080.. 4k is used when watching content, case closed.