403 Forbidden should be used for urls that exist but will show no result (no index.htm in a folder or similar) to anyone. 404 should be used for urls that do not exist.
Usually, if you ask for a folder with the / missing at the end of the url, the server responds with a 301 Moved Permanently with an additional Location-header redirecting to the URL with the / added - which the client (browser) usually answers with a request for the new URL.
See for example the following piece of HTTP communication (caught with Firefox's LiveHTTPHeaders extension [no http bodies], I emboldened the important pieces) - note the / or not / at the end of urls/url-parts:
http://www.andreas-waechter.de/IRL
GET /IRL HTTP/1.1 Host: www.andreas-waechter.de User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7.5) Gecko/20041107 Firefox/1.0 Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Language: en-us,en;q=0.5 Accept-Encoding: gzip,deflate Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive: 300 Connection: keep-alive