nvidia shield tv, best 4k player im aware of. nice, fast, android tv (so runs kodi, plex, spmc, emby etc).
I haven't tried the nVidia Shield, but I have read quite a bit about it with a view to potential purchase. It does 4K and most of the reviews seem pretty good. It also seems to play nicely with Plex, which is what I am also mostly using for content distribution.
Another vote for Nvidia shield.
I have loads of 4k files - incl with Atmos and DTS:X soundtracks and it plays these fine off my unRAID server via my Onkyo HT receiver - using SPMC.