I did the update a day after this was first posted, since I had a NVMe drive that was sitting idle in the DS918. I copied all my docker containers over, and launched them so my spindle drives could go idle.
Everything worked fine for week, however magically all my directories and files on /volume4 disappeared, and all the docker data was lost, without any notice. This was before the DSM update, so clearly something else happened that nuked the partition. The DSM update installed last night, and /volume4 is still there, so the changes persist updates, but all the volume data is lost.