Solution :
I had also faced the error related to port 4200 recently. I followed below ways to kill the progress on the port number 4200
If you are using Linux :
Then use the below command to fix it
sudo kill $(sudo lsof -t -i:4200)
You could also try below command:
sudo kill `sudo lsof -t -i:4200`
If you are using Windows :
Then you need to open your cmd as administrator and type below command in your cmd:
netstat -a -n -o
After that you need to find the port with port number 4200 by just right clicking on your terminal and then click on find, enter the 4200 in your "find what" and then click on the "find next": If you found that the port number 4200 is used by the pid 18932. Then you need to type below command in cmd:
taskkill -f /pid 18932