Earlier this year, Sony released new line of Blu-rays where you can find this “Mastered In 4K” and I’m sure you want to know what that actually means. I hope that in this short article I will answer all your questions and if not, feel free to add a comment.
So what “Mastered In 4K” means? Well, it means that these discs were created from video that was 4K Ultra High Definition, but these discs are normal HD quality Blu-rays. The 4K video was converted to classic HD so these videos will work in any normal Blu-ray players. They probably should be in theory higher quality because their basic source was higher quality, but they still have the same resolution as normal HD discs. So they are something like “super HD discs” they are still HD, but the the best possible quality of HD you can get.
Well the truth is that this is probably mostly marketing, because many Blu-rays already released as “normal HD” had Ultra HD video as source, you just could not find this info so easily.
They also claim that these discs are the best for upscaling if you already have UHD TV, that they even have some special upconverting protocols. I really do not know if that is true, but truth is that they really are the best you can get for upscaling to Ultra HD if you do not have any source of Ultra HD quality video.
It is not possible to have true Ultra HD video on classic Blu-ray disc, because its maximum capacity is about 50 gigabytes. Estimations are that average, more than hour long movie in Ultra HD resolution has at least 100 gigabytes. So you will need some other data storage than Blu-ray disc for UHD videos.
Here you can see comparison of HD Blu-ray from 2009 and Mastered In 4K 2013 Blu-ray movie Ghost Busters.