Please note the single bay My Cloud devices do not officially support Plex within the firmware/OS. The more expensive My Cloud units do support the Plex “add-on” third party module however in all cases they don’t officially support transcoding of media files.
The subject of 4K streaming has been discussed previously, see the following thread. The My Cloud should be able to stream 4K media provided one is using Gigabit connection/speed end to end on the local network. As also indicated in that thread the ability for the TV or end device to display or play 4K media is dependent on that device. In that thread one person indicated that their TV supports playing 4K via USB but not via DLNA Client/Ethernet.
https://community.wd.com/t/streaming-4k-from-my-cloud-to-4k-tv/155480
