The gap between 3.0 and 4.0 has been too large. The Horde team tries to keep backward compatibility within a major version. Probably nobody tried running a recent Kronolith from 2010 on Horde 3.0 from 2004. There'd probably be some issues and limitations but they should be minor.
While this kind of long term support may be useful for some edge cases it can also impede development progress. A good example are the Kolab drivers in the currently released version: Kolab support within Horde started to improve in 2006. At that time Horde3 was already the active branch and there was some very basic Kolab functionality in there. So the new code had to keep those interfaces stable. Which basically meant twisting and bending it into code that would do just that but otherwise be really problematic. That code is still a reality in the current stable release from Horde3.
This and similar problems did not pass unnoticed though and changes to the release cycle have been discussed internally for a while already. With the release of Horde4 approaching it now makes sense to discuss such changes with the Horde community. The envisioned target is a time based release cycle.
The discussion started today on the Horde development mailing list. Feel free to listen in or to add your own comments if you want to influence the direction of our future release mode.