If 4k is enabled this link lets you browse 4K UHD titles https://www.amazon.co.uk/gp/video/storefront?ie=UTF8&merchId=UHD. The app should have a 4k option as well. Everything in the chain needs to be port HDCP 2.2 for it to work.i haven't used i player in ages dont have a license. I cant see 4k mentioned anywhere on any amazon video page other than an faq. under qwuality its just "low medium high" catagories are sd or hd.
netflix has hdr and 4k options labeled.