Returning 404 Error ASP.NET MVC 3

I have tried the following 2 things to have a page return a 404 error:

public ActionResult Index()
{
    return new HttpStatusCodeResult(404);
}

public ActionResult NotFound()
{
    return HttpNotFound();
}

but both of them just render a blank page. How can I manually return a 404 error from within ASP.NET MVC 3?