How to add a redirect from a non-existent page to 404?

Question

htaccess

# redirect all requests to index.php loader
<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteBase /

  RewriteRule ^index\.php$ - [L]
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteRule . /index.php [L]

</IfModule>

Answer 1

ErrorDocument 404 /404.html