I would simply turn one of your pi4's into a "nas" among it's other functions. Use ZFS on top of a say... 4 USB3 enclosures with identical drives. and it serves out files over NFS. You could even get creative with it and use it as a persistent volume driver for k8s or swarm. You could even use an enclosure like this, but that is a little on the pricier side for a simple enclosure.