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).
High Efficiency Video Coding implementations and products
On September 5, 2014, the Blu-ray Disc Association announced that the 4K Blu-ray Disc (as known as Ultra HD Blu-ray) specification will support 4K video at 60 fps, High Efficiency Video Coding, the Rec. 2020 color space, high dynamic range, and 10-bit color depth. 4K Blu-ray Disc will have a data rate of at least 50 Mbit/s and may include support for 66/100 GB discs. 4K Blu-ray Disc will be licensed in the spring or summer of 2015 and 4K Blu-ray Disc players have an expected release date of late 2015.