Well... just to add some things to avoid the error.
As i started my programm in eclipse on my windows pc, it was working fine, but when i uploaded it on my virtual server with arch linux i got the 404 error.
So first of all make sure your Programm directory has the proper privileges. After this i saw my Programm in the Manager Tool... started, but still gave me the 404 finger.
my last error i found to make it work was the directory:
Wrong:
--exampleApp
----WebContent
------WebInf
--------web.xml
--------classes
----------dirToMyPackage
----------app.class
right:
--exampleApp
----WebInf
------web.xml
--------classes
----------dirToMyPackage
----------app.class
took me quite some time
hope i can hel sb
regards
PS: content of the hello world example i used can be found
here