It has been modified to work on OMV4. I posted instructions for users that wanted to build and install it somewhere on here. I am not sure where they are though. If anyone wants to look through my post history (I don't have time to do so right now), you should be able to find it.
Even though it was modified for OMV 4 and the latest code is in the repo, people are still having problems with it. And as technodadlife mentioned, the openvpn-as plugin hasn't been ported at all.