Apache Tomcat 4.0 was a ground-up rewrite designed for better flexibility and performance compared to the earlier 3.x versions.
While 4.0 was the initial release, users typically seek Tomcat 4.1.40 , which was the final stable "refactored" version of the 4.x branch, offering better performance and JMX-based administration. Key Features of the 4.0 Branch apache tomcat 4.0 download
First to support the Servlet 2.3 and JSP 1.2 standards while maintaining backward compatibility for Servlet 2.2 apps. Apache Tomcat 4
Included support for AJP 1.3 and the WARP connector for linking Tomcat to the Apache HTTP Server. users typically seek Tomcat 4.1.40