WordPress version 4.0.1 is now available for download. This latest version of WordPress is an important security update. If your site supports automatic background updates, your site should already by up to date – or will be within the next few hours. This security update is considered critical – as there are multiple cross-site scripting issues as well as 23 bug fixes in the update. Please take a few minutes to back up your site and apply this update now.

You can read more about the update: https://wordpress.org/news/2014/11/wordpress-4-0-1/

You can download WordPress 4.0.1: http://wordpress.org/wordpress-4.0.1.zip