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On Windows you need to use Microsoft Edge (not the new one, the one that ships in-box) and it will render in 4K

Using the netflix microsoft store app also does this

The Netflix app has additional benefits such as higher bitrate audio and multichannel 5.1 sound.

Also supports downloads and offline viewing.

I have a 100-minute subway commute and a Windows 10 laptop. The ability to download is nice. If I wanted to, I could watch two movies a day in 4k.