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
High-resolution displays such as 8K allow for each pixel to be indistinguishable to the human eye when viewed at a typical distance from the screen. 8K resolution can also be used for the purpose of creating enhanced lower resolution videos through a combination of cropping techniques and/or with downsampling techniques used in video and film editing. Resolutions such as 8K allows filmmakers to shoot in a high resolution with a wide lens or at a further distance, in the case of potentially dangerous subjects (such as in wildlife documentaries), by being able to zoom and crop digitally in post-production. The technique involves taking a portion of the original 8K image and cropping it to match a smaller resolution such as the current industry standard for high-definition televisions (4K, 1080p, and 720p).