There aren't many 4K videos uploaded to YouTube, but you can find some examples in this playlist. To switch to 4K, select "original" from the list of versions that are available. YouTube says that you need an "ultra-fast high-speed broadband connection" and it's probably a good idea to play the videos in fullscreen on a big-screen TV.
For now, the support for 4K videos doesn't mean too much, but it's nice to see YouTube pushing technological boundaries.