You cannot put anything in user.config except things from the appSettings section of Web.config, anything else you must maintain customizations in your Web.config and restore them to the new Web.config when you upgrade.

You could probably put that in your Web.config but I think it is not a good idea and could hurt SEO because with a redirect to an error page the status code served is not 404 but 200 and this means search engines will never see a 404 on your site, every unfound url will look like a duplicate page.

You could comment out the PageNotFoundHandler from the httpModules section of Web.config and let IIS serve the 404 page, and then do some googling about how to customize the IIS 404 page

Hope it helps,

Joe

Look for this in httpModules section:

<add name="PageNotFoundHandler" type="mojoPortal.Web.PageNotFoundHttpModule, mojoPortal.Web" />

and comment it out like this:

<!--
<add name="PageNotFoundHandler" type="mojoPortal.Web.PageNotFoundHttpModule, mojoPortal.Web" />
-->

You should make notes about this customization so you remember it when you upgrade later. It is important always to use the newWeb.config and re-apply customizations like this.