Symfony4 how to create a custom error page 404

You have to start by installing the necessary bundle:

composer require symfony/twig-pack

When done, create the template error404.html.twig

templates/
└─ bundles/
   └─ TwigBundle/
      └─ Exception/
         └─ error404.html.twig

Example of the error404.html.twig template

{% 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 test the rendering, check the configuration in config/routes/dev/framework.yaml

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

On recent Symfony versions, the route is declared in config/routes/framework.yaml

when@dev:
    _errors:
        resource: '@FrameworkBundle/Resources/config/routing/errors.php'
        type: php
        prefix: /_error

The error page is then available at http://localhost/_error/404