The migration from Raspberry Pi 3B+ to model 4 (4GB) went exactly as planned (and hoped):
ensured the system was totally updated, then just moved the SD card and USB drive over the new model 4 and booted. Everything worked perfectly!
Then this morning I noticed that automatic updates had updated NCP to version 1.16.1 and Nextcloud to version 16.0.4!
NextCloudPi is great, thank you guys !