You should not worry about admin 404 page. None of the front-end users should have access to that section. But if you really want to check it first of all upload a fresh system files to make sure that none of them are broken. Make sure system is up-to-date. If nothing changes that means that one of the extensions you have installed is breaking default system flow (i.e. doesn't let the system to handle the request). If you want to check default admin error page look at
/templates/isis/error.php (if you use default admin template).