Clustering was available in the previous Tomcat version (version 4.1) as a third-party JAR file; it wasn't very easy to install or configure to make multiple Tomcat instances run in a cluster. JavaGroups is a popular choice for adding clustering capabilities in open source servlet containers (Tomcat) and application servers (JBoss). But in the latest version of Tomcat server, clustering comes as part of the main installation package. This minimizes all of the extra effort that goes into integrating third-party clustering implementations into the Tomcat server.
However, i could not find a detailed explanation as to how to set it up. Reason why i am trying clustering with Tomcat4, is that we already have an application running on that version.