http://www.techradar.com/news/xbox-one-x-will-support-4k-streaming-on-amazon-prime-video-worldwide-at-launch

Planning on picking up an Xbox One X? You'll be happy to know that it will support 4K streaming through the Amazon Prime Video app when it launches on November 7. Nor will this feature be limited to US Xbox users. As of today, the Amazon Prime Video app is available globally on Xbox One.

In the words of Microsoft's announcement on Xbox Wire, this is true from "Australia to Canada and even Bhutan to Uzbekistan."