Ok, unplugged a drive while powered down and tried the command that ness1602 suggested. changed it a bit to only include dev b,c, and d as those are the remaining 3 in the array.
Code
root@openmediavault4:~# mdadm --assemble --force /dev/md0 /dev/sdb /dev/sdc /dev/sdd
mdadm: /dev/sdb is busy - skipping
mdadm: /dev/sdc is busy - skipping
mdadm: /dev/sdd is busy - skipping
No joy there.
I then went to the GUI under file systems where the RAID array shows up as a device and unmounted it. It changed to unmounted.
I tried the command again with the same results.