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
Japan's public broadcaster NHK was the first to start research and development of 4320p resolution in 1995. The format was standardized by SMPTE in October 2007, Interface standardized by SMPTE in August 2010 and Recommended as the international standard for television by lTU-R in 2012. Followed by public displays at electronics shows and screenings of 2014 Winter Olympics in Sochi and public viewings in February 2014 and the FIFA World Cup in Brazil in June 2014 using HEVC with partners AstroDesign and Ikegami Electronics.