how to run node js using domain

You dont assign a domain to a node.js server, instead you load your app onto a machine which has an ip adress, which in your case is 42.12.251.830:4000. You then need to make sure your app is listening on the correct port, which on most servers is 80

using express it’s as simple as

var app = express()
  , server = require('http').createServer(app)
  , io = io.listen(server);

app.get('/', function(req, res) {
  res.sendfile('./public/index.html');
});
server.listen(80);

now getting a domain name to point to this ip adress is an entirely separate matter. You need to make your name server point to the ip. Your name server will usually be the company you bought the domain name through, for instance GoDaddy is a Domain Name Server (DNS). So if you had a domain name with them, you would go on their site under DNS settings and change the ip adress. Your domain name will then point to your ip adress and should render your node.js app.