Node/Express: EADDRINUSE, Address already in use - Kill server

First, you would want to know which process is using port 4300

sudo lsof -i :4300

This will list all PID listening on this port, once you have the PID you can terminate it with the following:

kill -9 <PID>

where you replace <PID> by the process ID, or the list of process IDs, the previous command output.