custom pages 404 redirect and error display

I use the following to display asp code errors on the website.

Now, I want users to redirect to my 404.asp page when end up in a non existing page (aspx, asp, html, etc..).

What do I need to add? I tried different variations but couldnt success so far.

<configuration><system.webServer><httpErrors errorMode="Detailed" /><asp scriptErrorSentToBrowser="true" /></system.webServer></configuration>