UmbracoControllerRedirectTo404Page

ProductController.cs

public class ProductController : Umbraco.Web.Mvc.RenderMvcController
{
        public ActionResult ProductPageTemplate(RenderModel model, string productId)
        {
                if(productId.IsValidProductInDatabase())
                {
                        // Product is valid
                        return base.Index(model);                                  
                }
                else
                {
                        // redirect to 404 page      
                        // send HTTP STATUS CODE 404
                        var errorPage = new umbraco.handle404();
                        errorPage.Execute(HttpContext.Request.Url.PathAndQuery);
                        System.Web.HttpContext.Current.Response.Clear();
                        System.Web.HttpContext.Current.Response.StatusCode = 404;
                        System.Web.HttpContext.Current.Response.Status = "404 Page Not Found";

                        System.Web.HttpContext.Current.Response.TrySkipIisCustomErrors = true;

                        var page404 = Umbraco.TypedContent(myNodeIdContent404PageInt);
                        return View("NotFoundPage", page404);
                }
        }
}