I followed this tutorial http://symfony.com/doc/current/cookbook/controller/error_pages.html to customize twig 404 error page, I created the file "app/Resources/TwigBundle/views/Exception/error404.html.twig".
It shows up as expected, the only problem is that, I have a message injected automatically at the the closing html tag :
Not Found
The request URL /app.php/register was not found on this server.
I noticed that it was injected whenever my page contains the closing body tag and I can't get rid of it.
EDIT:
I found the solution, I was using Monolog, and activated another handler not the best way, which made the confusion and printed the error twice.