Hi! In this old thread , there's a link, but I don't know if "PRMVIDLNCH.apk" is the latest version.
When opening it, it prompt me to update the Amazon VOD app (even though the latest version is already installed, freshly downloaded from apkPure) but Google Play Protect told me that the download was suspicious, so I'm assuming that "PRMVIDLNCH.apk" has been tampered with.
Do you use other version of Will Blaschko's app or is there any newer method to run Amazon in a MiBox 3?
Not sure about the Mi Box 4, but on the 3 you can sideload a copy of Prime Video extracted from the SHIELD. Links are floating around this subreddit and the app is avaliable on Aptoide.