From Windows Central, dated July 2020 so not the most current but.
"Prime Video on Windows doesn't support 4K, HDR, or Dolby 5.1 surround sound from what I can tell. When set to "Best" quality, the app states that it's playing in HD. I don't see any audio settings for surround sound like there are in other apps."
The new Amazon Prime Video app for Windows has offline playback and scales well on PCs, but I don't love it. Here's why I think Amazon fell short with its Prime Video app for PCs.
www.windowscentral.com
From Amazon themselves they say nothing about 4K support, only up to HD for PC streaming
Prime Video is available via a web browser on a computer running Windows, macOS, Chrome OS, or Linux.
www.amazon.com
4K on the 13", what do you have the scaling set at? With my 17.3" laptop, I need it to be at least 200%, making 4K useless. If I could have had 100% AdobeRGB with a 1080p display instead of 4K I would have done it.