What is 4K? What is UHD?
4K and UHD (Ultra HD) refer to the resolution, or number of pixels, of the image captured and displayed. UHD is defined by ITU-R as an image resolution of 3840x2160. This is precisely four times the resolution of 1920x1080 and has a 1.78:1 aspect ratio. This is typically used for presentation on Consumer Electronics television sets and projectors. 4K is defined by Digital Cinema Initiatives as an image resolution of 4096x2160. This has an aspect ratio of 1.89:1 and is typically used for cinematic presentation.
These terms, often used interchangeably, describe a resolution which is 2x the width and height of full HD or 1080p. Instead of using the term "2160p," the industry has adopted the terms UHD and 4K. Netflix has adopted the term "UHD 4K" to make it simpler for our customers.
For a camera, this is determined by the number of photosites on the camera’s sensor. For a display, this is determined by the number of pixels on a TV panel or projector’s imaging chip.