All up and running with 4xCM4s and a SATA SSD on node 3. Runing Raspberry Pi Lite 64-bit and all happy as a clam.
The only hiccough was the well-documented 'use node 3 or 4 for USB initialisation' which I'd managed to read just prior to starting the set-up process, and also wondering why a M.2 SATA wasn't working in the Node 1 M.2 slot (no SATA capability, but can be done with a M.2 SATA adapter/controller (which I'm waiting to arrive)).
I also added a static MAC to the BMC so that my DHCP server can give it a static IP.
Now what do do with it.... Going to play with MicroShift (the edge-device version of OpenShift) to build a K8 cluster.
Also waiting for a case to arrive, and some heatsinks. So at the moment it's just a disco sitting beside me.