render a 404 page on routing error in rails

I'm trying to render the integrated 404 page in rails as an exception. I tried this but still getting the routing error page:

posts_controller.rb

def destroy
if current_user.username == @post.email 
@post.destroy
respond_to do |format|
  format.html { redirect_to posts_url }
  format.json { head :no_content }
end
else
  not_found
end

application_controller.rb

 def not_found
  raise ActionController::RoutingError.new('Not Found')
end 

routes.rb

 Booklist::Application.routes.draw do
 get "pages/faq"
 get "pages/about"
 devise_for :users
 resources :posts

 root 'posts#index'
 end

view:

<% if current_user.username == post.email %>
  <font color="red">This is your post! Feel free to edit or delete it. ->  </font>
    <%= link_to 'Edit', edit_post_path(post) %>
    <%= link_to 'Destroy', post, method: :delete, data: { confirm: 'Are you sure?' } %>
    <% end %>

No route matches [GET] "/posts/15/destroy"

Instead of

render not_found

you could use

render file: "#{Rails.root}/public/404.html" , status: 404

Or

render file: "#{Rails.root}/public/404.html" , status: :not_found

UPDATE

def destroy
  if current_user.username == @post.email 
    @post.destroy
    respond_to do |format|
      format.html { redirect_to posts_url }
      format.json { head :no_content }
    end
  else
    render file: "#{Rails.root}/public/404.html" , status: :not_found
  end
end  ## end is missing

UPDATE 2

If you want to display 404 error page in development environment then make sure that the following is set to false in development.rb file:

  config.consider_all_requests_local       = false 

WARNING: This also means that you would not see any errors raised on your application(stacktrace etc in view).