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.