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).
4K resolution
The resolution of is the dominant 4K resolution used in the consumer media and display industries. This is the resolution of the UHDTV1 format defined in SMPTE ST 2036-1, as well as the 4K UHDTV format defined by ITU-R in Rec. 2020, and is also the minimum resolution for CEA's definition of "Ultra HD" displays and projectors. The resolution of was also chosen by the DVB Project for their 4K broadcasting standard, UHD-1.