I can confirm that Amazon Prime is restricting resolution to 1080p HD on firestick 4k.
I have a 4k firestick and and Sony 4k android tv.
On 4k stick when watching War (hindi) it is beamed at 1080p and when using the Prime video app on sony android tv,
it is beaming at ultra hd. Same case with other ultra hd titles.
I have a 4k firestick and and Sony 4k android tv.
On 4k stick when watching War (hindi) it is beamed at 1080p and when using the Prime video app on sony android tv,
it is beaming at ultra hd. Same case with other ultra hd titles.