A Pi3 would work with OMV, but just that, it works. Don't start installing lots of plugins and wrappers on it. It is not powerful enough.
If you already have a Pi4 you don't need the Pi3. The Pi4 can do all the work, install OMV on it and whatever you need. The only doubt I have is HA on a Pi. I don't use HA but I have read other users saying that HA must be in a vm to take advantage of all its features.
I’m thinking of partitioning the drive, for my Mac Time Machine backup and the other one for OMV backup. What’s the best way to achieve this?
OMV will not make it easy for you to create/manage partitions. You don't actually need to do that. OMV is designed so that all configurations can be done through the use of shared folders. Just create a shared folder for each use you need.
Can you explain further please? Never used it!
Have you never used shared folders in OMV? 
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