[eluser]luismartin[/eluser]
Thanks everyone for helping. Sorry Roberto, I misunderstood your solution. I thought you meant you had to edit the show_404() function. Now I see I was wrong.
Anyway, I'm trying to figure it out... how do you get the necessary data (main menu, footer, etc) to display it in the modified default error_404.php page? At this moment I have just copied the output html of the view for the 404 custom controller, and replaced the code in the default error_404.php file. I have tested this way and it works. However this is a bad solution because it's just plain html and any change in the structure, a menu item for instance, will force me to copy and paste again the resulting html of the 404 error controller into error_404.php