I got DH the extended version, but it was years ago, I don’t know what a 4K disc is because techtard. I’ll suggest to him. Take his mind of wanting to be a ninja and go on a justice rampage.
4K is just the latest generation of video tech, like Blu-ray and high definition but one generation newer. To exploit it you need a 4K TV and, for playing the discs, a 4K disc player.
Whether it’s worth it depends on how much you watch and what you watch. But it can be truly spectacular, which LOTR is. 2001 is also a great 4K disc.