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It's possible that you are limited to lower resolution. Can you confirm whether you can get any of the 4k streams?

Widevine is required for any playback.

There is no browser support of 4K playback on Amazon at all, irrespective of the DRM situation.

Doesn't Brave have Widevine in it?

Brave downloads the Widevine EME module on demand. It's not included by default, but they have an agreement with Google to be able to include it.