First: Do you have the tomcat plugin in your pom.xml file?
Second: You need to remember that doing a Maven clean install does not start the tomcat server, that's why you are getting the 404 error.
You should launch your application by right-clicking, choosing Run-as, Maven build..., then enter tomcat7:run in the Goals text box (this assumes you have the tomcat7 plugin configured in your pom.xml file.
I'm assuming you are following an online tutorial. Which one is it?