OK, so here's what I did:
1. Got the PS5, set it up and connected via ethernet, logged into my PS account, updated to latest OS, shut it down.
2. Installed the 2TB NVME Samsung 980 Pro into the slot, rebooted and formatted it. All good.
3. Attached an external 4TB Samsung USB SSD drive to PS5, formatted and set for PS4 stuff.
4. Turned on my PS4 with internal 2TB spinny drive and migrated what I wanted to the PS5's built-in drive then moved to the 4TB External on the PS5.
5. Unplugged the 1TB external SSD drive from my PS4 and connected to my PS5. Rebooted the PS5 and it instantly saw the PS4 USB drive and disconnected the 4TB external PS5 drive automatically, I then moved all contents from the 1TB PS4 drive to the 2TB NVME drive on the PS5. That took 6 hours and when I woke up it was done.
6. Removed the PS4 external drive and rebooted the PS5 and now I have built-in SSD for misc, 2TB NVME for PS5 only games and apps, and 4TB external USB for all PS4 games and apps.
I just have to move some more stuff from the PS4 internal drive as when this all started I could not move it all due to storage constraints, but now I have more than enough.
Total of 6TB!!! I am happy!