I just did a quick TeamViewer session with @Zorn and found that even with a manual command-line install of the correct package would fail.
It appears that the WD OS5 somehow failed the first time to install PlexMediaServer-1.20.4.3508-6abbd541f-MyCloudPR4100_OS5.bin and then left a folder on disk which prevented the reinstall from working. It shows up in the Web UI as installed, but there is no app binaries in /mnt/HD/HD_a2/Nas_Prog/plexmediaserver
These were the steps I took to fix it.
- Check that
/mnt/HD/HD_a2/plex_confexisted and contained his App Data for Plex (Library, Preferences etc as this means that the migration had already happened. - check for
/mnt/HD/HD_a2/Nas_Prog/plexmediaserverdirectory and that it only contains a single file. - Once I knew what state it was in. I removed the file under the
plexmediaserverdir and then the bad directory.
rm /mnt/HD/HD_a2/Nas_Prog/plexmediaserver/*
rmdir /mnt/HD/HD_a2/Nas_Prog/plexmediaserver - I then installed the app again and this time it worked.
- We then had to claim the server again from the bundled Web App as he had removed the server from his account in an attempt to resolve the issue earlier.
If anyone else has issues please don’t remove/delete/change anything, and instead reach out here for assistance so we can get things up and running with minimal disruption to you 
Oh, and if anyone is having SSH issues, the username for ssh is sshd and is what you should use when connecting to the NAS via SSH.