We have learned now, in these years, as Apple delays, perhaps excessively in the introduction of some basic functionality. With the beta 4 of macOS, Big Sur and finally get the support for streaming in 4K on YouTube via Safari.
Thanks to the new update, all the videos in 4K available on Google’s android platform can be played with the default browser on macOS. Until today, in fact, to be able to enjoy that resolution on YouTube, it was a necessary third-party browsers supported, such as Google Chrome.
The reason of this lack was related to the absence of support for the codecs VP9, a gap that has finally now been remedied by the company from Cupertino with this new update. Unfortunately, there seems to be no intention on the part of Apple to introduce such a novelty the first time. You will then need to wait until this fall to finally be able to use YouTube in 4K resolution even on Safari.