FreeBSD 11.4 does have a newer kernel than 11.3. But the release notes don’t list any ABI compatibility changes. I imagine that’s what actually matters for jail/environment compatibility.
(And I’m not suggesting this has anything to do with Plex functionality.)
I added the line to start.sh. I am not using the packaging system to install plex in order to keep up with the latest plex release. I Just download the freebsd package, and run start.sh from crontab with user plex and defaults - library in /home/plex/Library.