The most recent YouTube app update for Amazon’s Fire TV has added support for streaming of 4k Ultra HD video, but there are a few caveats. The app will only support 4k on the second generation Fire TV player, must be connected to a UHD-compatible TV, and needs to be running FireOS 18.104.22.168 or higher.
The YouTube app is available from Amazon Underground, Amazon’s own Android app that provides access to download free apps from developers. The file size of the YouTube app varies according to device, but at most a 16.4MB file.
The update to the YouTube app was mentioned in a recent press release promoting Alexa improvements for Fire TV players. The updates include more voice command control with Alexa using the Fire TV Voice Remote including searching for titles through partner networks such as Starz and Showtime, finding local movie show times, and searching for local businesses and restaurants.