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).
5K resolution
As of 2016, the world uses 1080p as the mainstream HD standard. However, there is a rapid increase in media content being released in 4K and even 5K resolution. Online streaming services such as Netflix and Amazon Video launched videos in 4K resolution in 2014 and are actively expanding their collection of videos in 4K resolution. As 4K content becomes more common, the usefulness of 5K displays in editing and content creation may lead to a higher demand in the future.