Google has changed in recent weeks how it worksVideos in 4k format and currently cannot be played from the OS X Safari browser, But not only does man live on the Mac on Safari, although it is the browser that is best optimized, things as they are, hence many users who, despite its disadvantages, make use of it instead of others browsers.
The main reason YouTube videos in 4k format can no longer be played in Safari is because Google has changed the video codec used for playback, from H264, which is compatible with Safari, for VP9, an open source format that Apple does not seem to integrate into Safari.
The reason why Apple does not want to implement this codec we do not know and we may never find out, if finally Apple continues in its thirteen of not allowing its users to natively reproduce the videos in 4k format from YouTube in their browser, even more so when it is open source and you do not have to pay anyone to be able to add this function. Keep in mind that this limitation only affects videos that have been uploaded for a little over a week, all the previous videos can be played because Google used the H264 codec, as I mentioned above. To finish curling the curl, this limitation does not affect the videos embedded in web pages, as we can see in the following image of the same video in the upper capture.
So, the only way to be able to play YouTube content in 4k format, for all the videos that have been uploaded since the beginning of the year, is making use of the browsers that do offer support for this video codec, which are Chrome, Firefox and Opera. Again it is shown that Apple still does not pay attention to the needs or problems of users, since it forces us to have to install a second browser if we want to enjoy this content, something that does not happen for example with Microsoft Edge in Windows 10, the the only browser that also allows you to play content in 4k quality from Netflix.