As for 4k on macOS:
>The DRM Netflix uses for 4K content is the new HDCP 2.2 (High-bandwidth Digital Content Protection), which macOS does not support as of Mojave.
Also supports downloads and offline viewing.
They do support Widevine (at the lowest security level). When I last looked, with some hackery it was possible to force 1080p playback: https://github.com/vladikoff/netflix-1080p-firefox/pull/34