Long overdue, the Xbox One app finally support 4K video support in the latest app update.
Google has finally slammed us with a 4K patch on the Xbox One, allowing the app to support 4K videos and 60 FPS. Although unfortunately, no HDR support is included in the update. Xbox One S and Xbox One X consoles will be able to take advantage of this update.
You can install the update early from the Windows Store.