You didn't list the 404 error, and in particular the URL.
If you were hoping that servlets could be found just by typing their names, that was an insecure feature that's no longer supported in Tomcat. You now need to explicitly define the servlet and its relative URL in web.xml.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.
Looky! I'm being abducted by space aliens! Me and this tiny ad!
a bit of art, as a gift, the permaculture playing cards