4K is currently supported in our apps for the following devices:
Samsung Smart TV
Telstra TV 4700 Available in Australia
Roku Available in the US & Canada
If you're using a streaming device that's connected to your TV (such as a Telstra TV or Roku), your TV will also need to support 4K. Check with your TV's device manufacturer for more information.
If your device or TV doesn't support 4K, the stream will automatically play in high definition (HD) instead.
If you're watching via Amazon Prime Video (available in US and Canada), choose from the 4K or HD version in the episode or movie listing."