g1smd - 2:58 pm on Jan 3, 2013 (gmt 0)
If they exist, and people use them, then 404 is not appropriate.
The robots.txt will stop Google crawling them, but the URLs may still appear in the SERPs as URL-only entries.
If you want them out of the index, do not block with robots.txt but instead add the meta robots noindex tag on the page. They have to request the URL in order to know that you don't want it to appear in the SERPs.