How to redirect requested uri that don't exists to 404 page with Apache?

I've now used rewrite to achieve part of my goal:

rewriteengine on
rewritebase /
rewriterule ^([a-zA-Z][a-zA-Z0-9]*)$ resume.php?user=$1

How to add the 404 part?

You could just use ErrorDocument rather than mod_rewrite:

ErrorDocument 404 /yourUrl.html

You can use the Apache ErrorDocument directive to direct 404s to a particular URI.

For example:

ErrorDocument 404 /resume.php