4K resolution
As noted above, DCI 4K is 40962160, while 4K UHD is 38402160, producing a slight difference in aspect ratio rather than a significant difference in resolution. Traditional displays, such as LCD or OLED, are 3840 pixels across the screen, with each pixel being 1/3840th of the screen width. They do not overlap—if they did, they would suffer reduced detail. The diameter of each pixel is basically 1/3840th of the screen width or 1/2160th of the screen height - either gives the same size pixel. That 38402160 works out to 8.3 megapixels, the official resolution of 4K UHD (and therefore Blu-ray UHD discs).
Apple TV
The 1st and 2nd generation Apple TV video output can be set to either 1080i or 1080p; however, this resolution is limited to the user interface and the viewing of photographs – all other content is simply upscaled to those resolutions. Those models cannot play 1080i or 1080p video content (e.g., HD camera video). The 3rd and 4th generation Apple TV support 1080p video content. The Apple TV 4K, as the name suggests, supports 4K resolutions and HDR, including Dolby Vision. 4K content from sources such as iTunes can be played on a compatible 4K television set.