As of January 2017, here are the most popular apps that support 4K streaming:
-Netflix (shows like House of Cards, Marvel's Daredevil, Breaking Bad, Marco Polo, and more)
-Amazon Video (Shows like Grand Tour, Bosch, Goliath, and more)
-YouTube (4K movie trailers and channels like RocketJump, GoPro, Devinsupertramp, and more)
-Google Play Movies & TV (coming January 2017)
-Media Players like Plex, Kodi, VLC, MX Player, and the preinstalled Photos & Video app
Make sure your subscription account level support 4K streaming. For Netflix, this requires the premium account.