If you’re a plugin author, there are two important changes in particular to be aware of:
- TinyMCE received a major update, to version 4.0. Any editor plugins written for TinyMCE 3.x might require some updates. (If things broke, we’d like to hear about them so we can make adjustments.) For more, see TinyMCE’s migration guide  and API documentation , and the notes on the core development blog .
- WordPress 3.9 now uses the MySQLi Improved extension for sites running PHP 5.5. Any plugins that made direct calls to
mysql_*functions will experience some problems on these sites. For more information, see the notes on the core development blog .
Download: WordPress 3.9 RC1