AppleInsider has this article on what is going on. It seems that for all videos uploaded as of December 6, 2016 Adobe has moved from the paid to license H265 Codec to the free VP9 codec, and Safari does not support VP9. Previously the 4K videos had been encoded in H264, but both H265 and VP9 require much more power to playback and it will drain battery life, and use some serious processing power.
Obviously using a free encoder vs a paid license will help the bottom line for Google, as will videos that take up half the hard drive space, but users take a hit because people who don’t have the latest Machines will take a hit and be unable to playback 4K because it is so processor intensive. Seems a little soon, but Google has already made the switch.
If you want to encode VP9, Fnord has a free plug in for Adobe Premier Pro and Media Encoder.