It is still not known which way the huge amount of data for a movie in 4K quality will take to our 4K screens. There is also no broadcasting of 4K in the wild and it is still open, when it is about to come. That is why most consumers will watch material in HDTV quality, either 720p or 1080p on their 2160p television. But how does that work? The answer is upscaling.
Scaling the popular video formats 720p or 1080p to the 4K resolution is simple. A 1080p video source can be scaled to 4K perfectly through doubling every pixel horizontally and vertically. Every pixel of the 1080p video source will the be four pixels an the screen. Also 720p video format sources can evenly be upscaled by tripling every pixel horizontally and vertically.
Because the HDTV material can be easily upscaled to 4K, the pictures look better than the same material on a full HD screen.
A very common use of 4K upscaling is for movies mastered in 4K, which are available in Full-HD resolution on Blu-ray discs but were mastered from a 4K source.