Just a warning for others to remind/alert that MQTT brokers commonly use port addresses starting with 188x and if you add in extra node-red alternate testing installs - its easy to come into conflict with them - like what I did!
So on my Pi4 - I am developing in an alternate install that uses 1883 (I've already got other testing installs running on ports 1881 and 1882)
That is my first error, because if I was to add in an mqtt broker, it is going to default to 1883 so I'm recommending never using that port for Node-RED
Moving on, I wanted a test alternate install so I set up one on port 1884 but kept getting port in use in the log - couldn't understand why - so rebooted and it came back up with my main NR on port 1880 - nothing else running (or so I thought)
Launched my alternate install on port 1884 - port in use again.
Eventually, the penny dropped - I'd installed a test broker using the Aedes node and had set it to use port 1884!
So, I think in future, I'm going to use 1888x for alternate installs