While 4K adoption hasn't really gone mainstream yet, several providers have already jumped on board, Netflix most notably. On our forums, Canuck27 asks which 4K media apps are out there for Xbox One, and we'd like to get your input!
As for me, living in Europe, Netflix is the primary option for 4K streaming content, since Microsoft's own Movies & TV store is sorely lacking in this department. We've heard some rumblings that the situation may change in the near future, however, with more work being done to get 4K content on the platform.
For now, jump in the forum thread above and share the media apps you're using on the Xbox, day to day.