< evil voice>It’s alive!</evil voice>
In the end it was a combination of problems which were solved, in part, by my liver and some sleep.
It’s been a long week and I had perhaps to many gins after coming home yesterday.
- Install Buster onto RPi4
- Install Zulu as per this post
- Install OH 2.4 as per this document
- Install samba as per this document (see note below)
- Restore the backup from your other pi.
- wait for around a minute before trying to connect.
It certainly seems to start the service quite quickly. I’ve not timed it against the other but my rpi3+ takes a number of minutes before it settles down and behaves. Will see how this one works.
Some downsides which will cause rework for me -
You cannot boot from USB3 at the moment on buster. That is coming soon but not yet.
I bought a Sandisk U3 card - it has a read of c.110MB/s and write of c.70MB/s. The USB disk, a mSATA in USB3 caddy has a 300/330 rw 
How much that will matter in real life? Dunno. I’ll do it though for longevity.
I’ll port over the peripherals today and run this as a production system for the next week or so. Updates to follow
For the record, this is my current RPi3
You can see, memory was a problem…