Amazon Prime Instant Video has started streaming select films and TV shows in 4K resolution on compatible TVs.
Unlike Netflix's pricing structure, Amazon Prime Instant Video 4K content won't cost you any extra, all you'll need is a compatible 4K TV that has the Prime Instant Video app.
4K content offers four times the resolution of Full-HD so unless you've got a 4K TV or 2K smartphone you're unlikely to notice the difference on a regular TV.
According to IGN the first films to get the 4K treatment include 'Hitch', 'Crouching Tiger Hidden Dragon' and 'Philadelphia'.