From the WordPress 4.9.4 release post: This maintenance release fixes a severe bug in 4.9.3, which will cause sites that support automatic background updates to fail to update automatically, and will require action from you (or your host) for it to be updated to 4.9.4.
WordPress 4.9.4 contains 1 bug fix.
Detailed list
Upgrade/Install
- #43235 – WP Automatic Updates broken in 4.9.3.
List of Files Revised
wp-admin/about.php
wp-includes/update.php
wp-includes/version.php