There are a bunch of factors that affect whether Netflix will give you the highest resolution available for your hardware. Some of them are "soft" constraints, like the user agent, the pixel resolution of your screen, and what profiles the client requests from the server. Then there's hard constraints like the DRM version, HDCP level, and what codecs your hardware actually supports.

This extension tries to spoof HDCP status and codec support which is stupid and will not provide any benefit, since it is ultimately enforced by hardware.

But it also patches Cadmium to request a custom set of profiles which is useful and can improve compatibility: https://github.com/Pickle-Pixel/netflix-force-4k/blob/72e179...

For example, here's a set of profiles that makes 1080p work on Linux, as opposed to a mere 720p: https://github.com/DavidBuchanan314/Turbo-Recadmiumator/blob... (or at least it used to, I haven't tested it in ages)