Their "System Requirements" doc says 4K is only available on Windows, and only with Edge:

https://help.netflix.com/en/node/23742

It is possible that doc isn't kept up to date though. :)

I don't want to advance anything (don't have time to find the source), but I think Apple have something special built in Safari/Webkit for Netflix. That relate to decoding, quality and battery combustion while streaming.

Safari & Edge support a more advanced version of HTTP Live Streaming (it's the same tech Apple uses for their event live-streams) plus the DRM scheme Netflix uses.