Tomcat 4 supports Servlet API 2.3 and JSP 1.2. You need to make sure that you're not using any servlet/JSP features introduced later. A starting point would be to compile your servlets against the Tomcat 4 libraries instead of the Tomcat 6 ones, and also to not use the JSP EL. A surer alternative would be to install Tomcat 4 and develop against that.
What do you have to say for yourself? Hmmm? Anything? And you call yourself a tiny ad.
a bit of art, as a gift, that will fit in a stocking