richarst,
The error is probably saying something about “JDK”, if it is, then you need to go back to your Tomcat 4 settings, go to the JDK portion of the apps, and hit the “Add” button to add a new configuration for a JDK (you cannot use a JRE, which is what Eclipse defaults to using), so point it at something like C:\j2sdk1.4.2_04, or your equivalent JDK install dir.