There are some pretty substantial improvements in this version of Habari. We've improved the installer, so that more people should have less trouble installing the software; and if they do have trouble they should hopefully get useful information as to why. We have some great new media management solutions, including an integrated Flickr browser, as well as the ability to record Viddler videos directly from within your Habari installation. There are obviously a lot of under-the-hood improvements to the code, as well as a strong foundation for a robust permission system (I've been working on this aspect quite a lot lately, and I'm eager to see it come to fruition in the 0.5 release!). You can review the 0.4 release notes for the overview of important changes, or peruse the changelog for a complete list of modifications.
This release represents the hard work and contributions of a lot of people! The Habari community has grown considerably in the last few months, as more and more people take it for a test drive and then help us smooth out the rough edges. I am delighted that so many people are taking the time to contribute to Habari, and it is this, I think, that is Habari's greatest improvement thus far.