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).
Graphics display resolution
This resolution, sometimes referred to as 4K UHD or 4K2K, has a 16:9 aspect ratio and 8,294,400 pixels. It is double the size of Full HD (19201080) in both dimensions for a total of four times as many pixels, and triple the size of HD (1280720) in both dimensions for a total of nine times as many pixels. 38402160 was chosen as the resolution of the UHDTV1 format defined in SMPTE ST 2036-1, as well as the 4K UHDTV system defined in ITU-R BT.2020 and the UHD-1 broadcast standard from DVB. It is also the minimum resolution requirement for CEA's definition of an "Ultra HD" display. Prior to the publication of these standards, it was sometimes casually referred to as "QFHD" (Quad Full HD).