They Say What They Say

Okay, so the third season of due South is about to (finally) come out on DVD, however it only goes up to Mountie on the Bounty parts 1 and 2. That's all very well and good. However, what I want to know is, with all the rigmarole they had to go through for this release, how long do we have to wait until season 4 comes out on DVD? Another 2-3 years? Why couldn't they just do the whole thing at once and save us (the people who will go out a buy the damn thing) a lot of pain and anguish as we wonder when the rest will appear?