Symfony4 how to create a custom error page 404

You have to start by installing the necessary bundle:

compose require symfony/twig-pack

When done, create the template error404.html.twig

templates/
Bundles/
   TwigBundle/
      Exception/
         Error404.html.twig

Example of template error404.html.twig

'% extends 'base.html.twig' %'

'% block body%'
    <h1>Page not found</h1>

    <p>
        The requested page couldn't be located. Checkout for any URL
        misspelling or <a href="{{ path('homepage') }}">return to the homepage.</a>
    <p>
%endblock %

To be able to test the rendering of the page, control that configuration in /config/routes/dev/framework.yaml

_errors:
    resource: '@FrameworkBundle/Resources/config/routing/errors.xml'
    prefix: /_error

When this is done, one can reach the error page as well:

http://localhost/index.php/_error/404