Did you remember to install Mosquitto on your Rpi4?
Mosquitto is an external service. It will not be part of openhab-cli backup. So you have to enter openhabian-config and install Mosquitto (or do it manually from command line).
If your Rpi do not have the same IP as your old Rpi, then you´ll need to change each MQTT client to the new IP as well. This is an “issue” I just rasied in another thread about openhab-cli, (not regarding MQTT but others. It will comply to MQTT as well though).
