Ragnar from Red Thread Games just posted an update about the upcoming Unity 5 version (with performance and stability fixes) and Book Four over on Kickstarter.
You can read the update here:
https://www.kickstarter.com/projects/redthread/dreamfall-chapters-the-longest-journey/posts/1390566
The short version?
The port from Unity 4.6 (our old game engine) to Unity 5.2 (our new game engine) has not gone as smoothly as expected, and as a consequence Book Four has been slightly delayed.
We don't have a final release date just yet, but as soon as we do, we'll post another update here.
The Unity 5 version of Dreamfall Chapters will be available via the beta branch on Steam within the next week, so if you want to help us test it and/or check out the performance improvements, stay tuned for that.