There some issue with digital ocean server.
I can start server with pm2 start app.js
monyrestparameters"
[PM2] Applying action restartProcessId on app [app](ids: 0)
[PM2] [app](0) ✓
[PM2] Process successfully started
┌──────────┬────┬─────────┬──────┬───────┬────────┬─────────┬────────┬─────┬──────────┬───────────┬──────────┐
│ App name │ id │ version │ mode │ pid │ status │ restart │ uptime │ cpu │ mem │ user │ watching │
├──────────┼────┼─────────┼──────┼───────┼────────┼─────────┼────────┼─────┼──────────┼───────────┼──────────┤
│ app │ 0 │ 0.0.0 │ fork │ 23428 │ online │ 5 │ 0s │ 0% │ 4.7 MB │ dev2rt987 │ disabled │
└──────────┴────┴─────────┴──────┴───────┴────────┴─────────┴────────┴─────┴──────────┴───────────┴──────────┘

0|app | server is listening on port 4000

but when I open server from browser. Request dont come to the server. I checked 4000 port its open and listening on the server.

For anyone having this issue, I solved it by appending some required node parameters.

Mine looked like this: pm2 start app.js --node-args="--harmony-destructuring --harmony_rest_parameters"

You may need to pass in some required arguments. Check package.json for a start script.

http://pm2.keymetrics.io/docs/usage/quick-start/#42-starts

I compiled my app.js using npm build then pm2 start public/app.js worked for me

instead of the command: pm2 start app.js
try this command: pm2 start ./bin/www

you should use the same line after the pm2 start as you have in the start script of your package.json file in the express app root folder