It appears that as of December 6th, YouTube stopped serving H264 versions of newly uploaded 4K videos. Instead the high resolution videos are only available in the VP9 codec developed by Google, a codec that Safari does not yet support.
Reddit user themcfly writes:
From the start of December, new uploads from Youtubers that I follow who always uploaded 4K (MKBHD, Unbox Therapy, LinusTT etc.) were showing up only in 1440p on Chrome. Disabling H264fy made 4K show up again. So, somehow, Youtube has stopped storing/serving H264 4K version of videos on their servers (maybe as a storage space saving measure?) so if you are watching from a browser that does not support VP9 (like Safari) I guess you are out of luck. I hoped it would be a temporary issue but it has been a month now. I am stuck on using Chrome and seeing my i7 CPU going up to 50-60% usage for a simple video play, quite embarrassing from Google part since VP9 hardware support is still quite limited to only the newest GPU chipsets, so on most of 2016 hardware and earlier the CPU only has to do the heavy lifting.
YouTube has yet to confirm the shift; however, we are noticing the same thing. Last year, Google said "VP9 unlocks the burgeoning world of 4K videos. At larger video sizes, VP9 actually gets even more efficient than its predecessors, so uninterrupted 4K content can now be streamed by a significant and growing part of the YouTube audience."
Read More [via Wieland]