attaching helloWorld.ear did not work... sorry
I managed to solve the problem on my own. :)
The "<servlet-mapping>" in %JBOSS_HOME%/tomcat4.1.12/conf/web.xml for the servlet "invoker" was commented out. This servlet loads other servlets and is by default mapped to "/servlet/*" on Tomcat4 (without jboss). Now it's working and it invokes my servlets...