NodeBB will run perfectly fine on node versions as low as 0.10, there is maybe only a very smaller number of third-party plugins that may require node 4.
You have installed a Node Version Manager called n which does indeed make it easy to change node versions. n does this by copying the files from it's directory to /usr/bin/ when you use it's commands. The reason you had to do this manually probably has to do with a permissions issue.
You are running all your commands as sudo, when none of them should be run with sudo, this can cause permissions issues and is possibly a security risk. Unfortunately, it may be too late to correct this problem. Next time, check here if you run into any issues when not using sudo.
I've been trying to do this. I used git add config.json before pushing and that worked. Started the app from the console. It starts, connects to the db, says NodeBB is now listening on: 0.0.0.0:58466 Great.
But I'm stuck there, I do heroku open and no dice. Application Error. Logs say App crashed, code=H10
I don't understand this heroku thing. Maybe you'll have better luck after pushing that config.json.