http://www.engadget.com/2015/04/06/how-youtube-uses-vp9-to-help-4k/t's one thing to find 4K video, but it's another thing to play it -- the bandwidth needed to play high-quality 4K video could easily crush many home internet connections, let alone your mobile service.
YouTube isn't taking that challenge lying down, though. As the service explains, it has been encoding many videos in its newer VP9 format in recent months to make 4K more viable and improve the picture you see. The codec uses as little as half the bandwidth as the H.264 standard you see on many parts of the web, even as it bumps up the image quality by prioritizing sharp features and taking into account fast-moving elements in the footage, like water spray. The result is video that not only starts playing sooner (since it spends less time buffering), but runs at resolutions that your connection might not otherwise handle.
Even if your internet access isn't up to handling 4K, this could still make the difference between watching in HD versus blocky standard definition. The real question is whether or not your favorite apps and devices can handle it. Chrome, Firefox and a lot of recent hardware (such as the Galaxy S6) have native support, but there's no guarantee that it'll be ubiquitous -- not with the industry's H.265 standard competing for attention, anyway.