There are a couple of basic reasons why someone might want to use 4D in conjunction with another web server. If you are talking about using 4D's built-in web server along with another web server the most common reason would be for maximum performance. Serving static pages from WebSTAR, for example, will probably be faster than running them through 4D because that way you can reserve the 4D web server for generating dynamic pages and WebSTAR for static pages. An automatic, cheap form of load distribution (which means spreading the "load" around multiple servers).
4D can also be used as a CGI for another web server, which makes it easy to pass requests "sideways" to 4D. In this scenario the web browser would not communicate directly with the 4D built-in web server but instead only with the other web server. When a request for something from 4D comes in, the other web server sends the request to 4D - catches the response - and sends it back to the web browser. This allows another possibility which is higher security since 4D can be placed behind a network firewall to prevent any unauthorized access. Only the primary web server can "touch" the 4D that is protected in this way - thus preventing any possibility of being hacked into.
There are many choices for setting up communications between two (or more) web servers including 4D, especially including CGI configurations. Reasons for doing this sort of thing usually center around performance, security or compatibility with legacy systems.
Command Web Services, Using CGIs