Thank you for your comments.
I installed Sonarr using
wget -qO- https://raw.githubusercontent.com/Sonarr/Sonarr/develop/distribution/debian/install.sh | sudo bash
(or whatever was applicable for v.3).
I think that I’ll create a full backup on a Live-USB just before updating to v.4. Worst case scenario is that I’ll re-install MX Linux by restoring the whole system.
