404 Page response code

Hello Fourm, 
I got a single page website where everything is directed to using the custom error settings in the 404 page. It does a number of checks and gets the right content, all works fine :-)
I want to know how to send a 404 header response code back to the browser if the page requested is not found in any of the content locations.
I am using a proxy server and also checking the trace information, when trying various options.
I have tryed

Response.StatusCode = 404 Response.StatusDescription = "page not found"---------context.Response.Status = "404 Not Found"----------Response.Status = "404 Not Found"---------------Response.Clear() Response.StatusCode = ...