It is still questionable how 4k content should be played on ultra hd televisions. But whoever is able to play native 4k videos on his television or computer, should try out the official 4k Resolution playlist from YouTube to feel how it is like. Although YouTube allows a maximum resolution of 4096 × 3072 pixels (12.6 megapixels), the web-based video player is limited to 2048 × 1536 (2K, 3.1 megapixels). To view the full size videos, you will have to download the original video using a YouTube downloader of your choice. Of course, the videos will take a while to load, so grab a cup of coffee and enjoy great sharpness and details.
You can find the playlist here.