The nVidia shield Pro looks like a great machine. It does have Plex built in I see, so we can all agree it is made to play 4k content. Is that a stable device?
Unfortunately, no. The device is rock solid, but the plex client on Android TV devices has some serious issues that have plagued it for the past 8 months and plex has been slow (AKA, they haven’t) to address them. If plex ever does, it will be the best streaming device for plex again, like it was for years.
See: Infinite buffering when opening files on Nvidia SHIELD player (and other issues)