It doesn't work on Firefox. It appears not to work on Chrome. The suggestion is to use Edge, which on Windows already gets 4K support in Netflix anyway.
Here's a 4K enabler that only enables 4K where it's already enabled.
I understand it spoofs all of the checks it can, but the only Chromium browser that supports Widevine L1 (a requirement for 4K) is Edge, so even if all of the check spoofing works, it still won't do 4K.
There's even a table in the README that describes this exact scenario.
> If you're paying for 4K but using Chrome, Firefox, or a setup Netflix doesn't "approve," you're stuck at 1080p or lower. This extension fixes that.
It appears to only be useful on Edge on Windows.
The problem here is requiring hardware-attested DRM: Widevine L1 on Edge on Windows, and Apple FairPlay on Safari on MacOS. The only way to get hardware attested DRM is via browser specific (i.e.: native code) support that interfaces with the OS & GPU drivers. You can't get there through an extension.