We would also like to return 404 when the file is not found, because it seems Google needs it for things like Google Sitemaps.

What would be the correct way to return a 404 error code by default when requesting a non existent page?

For example, if you request something like:
http://openacs.org/sadfasdfasfd
You'll get a strange page with comments from nowhere and a 200 OK error code.