Apple Macintosh with T2 Security chip is needed to play 4K Netflix with macOS 11.0 Big Sur according to info on Netflix’s help pages.
“Netflix is available in Ultra HD on Mac computers. To stream in Ultra HD, you will need:
A Mac computer with macOS 11.0 Big Sur installed.
The latest version of Safari browser
Select 2018 or later Mac computer with an Apple T2 Security chip
A 60Hz 4K capable display (with HDCP 2.2 connection if external display).
NOTE: Every monitor connected to your computer must meet these requirements to successfully stream in Ultra HD.
A plan that supports streaming in Ultra HD. You can check which plan you’re currently on at netflix.com/ChangePlan.
A steady internet connection speed of 25 megabits per second or higher.
Streaming quality set to Auto or High. More information about video quality settings can be found in our Playback Settings article.”