WordPress 4.0.1 is now available. This is a critical security release for all previous versions and we strongly encourage you to update your sites immediately.
Sites that support automatic background updates will be updated to WordPress 4.0.1 within the next few hours. If you are still on WordPress 3.9.2, 3.8.4, or 3.7.4, you will be updated to 3.9.3, 3.8.5, or 3.7.5 to keep everything secure. (We don’t support older versions, so please update to 4.0.1 for the latest and greatest.)
More information here