So I got OMV 4 working on rpi4 using the upgrade instructions I posted earlier in this thread (upgrade 4.x stretch image to buster)
This is the wrong way and will break anyway. It needs 'apt pinning' to get RPi 4 support on Debian Stretch as shown already (there exists an OMV4 image for RPi 4 on transfer.sh which will disappear automatically within 12 days: New approach for Raspberry Pi OMV images)
I won't continue contributing to OMV until the @crashtest problem is resolved. If a project encourages totally incompetent people to mislead inexperienced users I don't want to be part of this project.