4k video is four times 1080p (the highest high definition setting on that YouTube bar). It also might be the future of TV technology—at least now that we know that it definitely isn't 3D. If your computer can handle it, hit "Original" on the settings bar and watch these ink drops fall in the nicest HD if you've ever seen. If your body can handle it, take mushrooms while doing so.
[H/T: Laughing Squid]