{% extends 'admin/layouts/default_sp.twig' %}
{% block title %}{{ _('404 Not Found.') }}{% endblock %}

{% block content %}
    <header><h1 class="in_menu sh_heading_main_b"><span class="h1_title">{{ _('404 Not Found.') }}</span></h1></header>
    <p>お探しのページは存在しません</p>
{% endblock %}

Convert to Twig: SP's /admin/comments/error404.twig
Convert to Twig: SP's /admin/comments/error404.twig
