Just got 6 gen apple tv and No 4K format available. No issue with gen5.
Gen 5 has no problem connected to Denon to Sony XBR900F. Gen 6 only shows 1080p and lower. Tried getting high end Audioquest HDMI 8k/10k cable connecting directly from Apple TV to Sony with no luck. Tried unplugging the TV for awhile (comment from Reddit) and still no 4k setting. The video setting screen is also lacking other option settings. At this point I'm of returning the Apple TV.