The local loop-back for a TCP/IP connection (for any TCP/IP application) is 127.0.0.1
This allows you to test your 4D application without an internet connection.

Launch your 4D web Application.

Start web-serving in 4D.

Launch your browser.

Type 127.0.0.1 as the URL to connect to.

You are now viewing your 4D web application in your browser.