мне вчера помогло вот это из stackoverflow:

The error 404 appears when Tomcat can't find the localhost.ser file. In order to get rid of this follow these steps: 1) In Eclipse, right click on server --> Properties --> Click Switch Location --> Apply-->Ok (This will switch the [workspace metadata] location to the installed Tomcat location.) 2) Then go back to server, double click it. This will open Overview tab. Under this tab goto -->Server Location --> Select Use Tomcat Installation combo box.

Now close it, save it and try run your server and then rerun the URL.

то же самое есть в первом уроке вот тут:
http://javabrains.koushik.org/2011/04/jsps-and-servlets-tutorial-01-setting.html