Try adding this rule to the top of your htaccess:
RewriteEngine On
RewriteRule ^404/?$ /pages/errors/404.php [L]
Then under that (or any other rules that you have):
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-l
RewriteRule ^ http://domain.com/404/ [L,R]