zotan: TBH, when one of my many devices is more than one update behind, I tend to just flash the latest version to save time.
On the one that is my daily driver I would restore the backup, if it's one of the older devices I keep for testing or sentimental reasons, I no longer bother restoring the backup.
One caveat when flashing ≥4 is that Jolla changed the username from "nemo" to "defaultuser", so restoring your backups from 2.x on 4.x will crap out (simply because the backup tool was not adjusted after the username was adjusted for fresh installs). Those that upgrade (instead of reflash) retain their old username of nemo.
Quick and dirty fix, is to symlink /home/nemo -> home/defaultuser after flashing your tablet but before attempting to restore your backup.