Here is the output of dpkg -l | grep openmediavault
I got virtualbox working ok by installing the backports using apt-get install -t stretch-backports linux-headers-amd64
and then installing omv-extras from the command line.
All the other plugins seem to have made it through the omv4 upgrade fine... they are just not showing as installed in the plugins list.
Ie syncthing, snapraid, plex etc are all running