As far as I know (I don't own a 4K TV) both KODI 16.1 and SMPC should be able to playback 4K videos at 4K resolution without any add-ons. It's the GUI only that shows 1080p, but on playback it should change to 4K.
Tho it's suggested to install the KODI frame rate patch for proper frame rates.
But you will get no HD audio at all and get aspect ratio trouble with HDR videos.
Other solution is the latest version of MX Player, it has been reported recently, that it hasn't got the HDR aspect ratio trouble present.