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).
Vudu
Movies are encoded in MPEG-4 video and Dolby Digital Plus audio. Vudu offers movies in three formats: standard-definition (480p and 480i), high-definition (1080p and 1080i) and ultra high-definition (2160p) or commonly known as UHD or 4k. Standard-definition movies are up-scaled to higher 1080i/p resolution and high-definition movies can be output at 1080p24. Audio is sourced in Dolby Digital Plus and output in Dolby Digital 5.1, with the option of stereo output. Audio is limited to stereo playback on HTPC/PC/Mac platforms. Users are able to adjust video to display in stretched, boxed, or zoomed format as well as adjusting overscan settings depending on their television format.