Ceph recommends 1gb ram per tb of storage. Going lower gives massive hits to performance and the 1gbe ethernet connection becomes a bottleneck with fewer nodes, so I don't think it's viable to scale this way
True, the Pi 4 with 8GB RAM would allow for 8 TB max recommended storage per node. Thus, requires you to have 7 Pis for 56 TB. I think that's still a good setup, even with 1GE.
btw, 10GE is the minimum requirement for a non-hobby ceph cluster