404_override and show_404

Hi guys, this is my frist post in CI community although i use this framework from a couple of years.

I think that if in route config file i change the default controller for 404 error, when i call the function "show_404" this function should call my custom 404 controller as setted in config file instead of simply load the default CI error view.
I m wrong?

Sorry for my english and thanks!