Bottom line: The next major iteration of macOS (Big Sur) brings with it 4K HDR for Netflix. Unfortunately, it will only work in Macs manufactured in 2018 or later. This limits the feature to 2018 or later MacBook Airs or Pros, 2018 Mac minis, 2019 Mac Pros, and 2020 iMacs and iMac Pros.
The problem appears to stem from Apple's security chip. According to Netflix's support pages, only Macs with the T2 SoC introduced in 2018 will process the 4K HDR signals, although it didn't say why. Older models with the T1 or no security chip at all will not be able to play Netflix in 4K.
It is likely because the T2 chip is more advanced and handles many more functions than the T1 could. The T1 was mainly responsible for processing and encrypting fingerprints for TouchID. It also managed access requests for things like the microphone and camera.
In contrast, the T2 SoC handles encrypted keys, system functions, including camera and audio control, drive encryption, "enhanced image processing" for Facetime, and perhaps most telling, HEVC transcoding. Indeed, the current product page for the iMac states:
"The T2 chip also enables powerful features like the 1080p FaceTime HD camera, face detection, enhanced speaker bass response, and "Hey Siri" — and makes transcoding HEVC video up to twice as fast as the previous generation." [emphasis mine]
If Netflix uses the HEV codec, it might explain why the T2-based systems are required, but this is just speculation.
Users will also have to have a Premium subscription, which runs $16 per month. Browser choice is not an option either. Netflix says its 4K streams will only run through Safari.
Image credit: sitthiphong