If you want 4K transcoding then I would really suggesting either building something or shelling out the money for a prebuilt server of some kind. 4K to 4K is not possible as Plex has not yet added the ability to transcode HEVC to HEVC. You can though transcode 4K to 1080p and is going to be 99% of what transcoded 4K ends up as anyway. 4K transcoding is notoriously difficult and takes a good bit of horsepower to do. You can get some NAS devices that are capable of it by adding in a small GPU, but transcoding 4K is still very much a pain in the butt. That article is a good read. Let me know if you have any questions about it and I am more then happy to answer them and continue to help you down this path.
