The Netscape Proxy Server version 3.x was essentially a coupling between the Netscape browser client, responsible for talking to remote servers, and the Netscape web server 4.x, which in turn handled the duty of talking to a client.
Sun Java System Web Proxy server 4.0 resulted from a major rewrite of Netscape Proxy Server in 2003-2004, when Sun decided to reinvest in Proxy technology.