The download contains the full 1.4 update and is intended to "prepare server admins and mod makers." Pretty Scary will fix a number of bugs and include new features, such as a three-headed boss called the Wither, mobs, wearable heads and more.
Update 1.4 will go live on Oct. 24. The patch has been in development since the release of 1.3.2, which released on Aug. 16. For a look at the complete list of updates in 1.4, check out the community wiki.