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).
8K resolution
, few cameras had the capability to shoot video in 8K, with NHK being one of the only companies to have created a small broadcasting camera with an 8K image sensor. By 2018 Red Digital Cinema Camera Company had delivered three 8K cameras in both a Full Frame sensor and Super 35 sensor. Until major content sources are available, 8K is speculated to become a mainstream consumer display resolution around 2023 as mentioned in UHD forum Phase-B recommendations. Despite this, 
filmmakers are pushing demand for 8K cameras due to their ability to capture better 4K footage.