Hi,

You can use this module

https://github.com/Geta/404handler

There's no need to install the Geta handler if you just want 404 pages and not CMS controlled redirects.

Turn custom errors off

  <customErrors mode="Off" />

Configure HTTP Errors pointing at a CMS path

<system.webServer> 
  <httpErrors errorMode="Custom" existingResponse="Replace">
      <remove statusCode="404" />
      <error statusCode="404" responseMode="ExecuteURL" path="/NotFound" />
  </httpErrors>
</system.webServer> 

Setup a controller for the error page that returns a 404 response in the controller action

Response.StatusCode = 404;