James Mirtle posted a while back that it isn't realistic to expect 4 teams from 1 division to make the playoffs. Tom Benjamin responded that: "I do agree that the most probable outcome will be two or three playoff teams from each division, but I do think four teams making it from one division will happen more frequently than he thinks"
I was under the impression that the divisional schedule would significantly effect the chance that 4 teams make the playoffs, but now in two seasons we've had 4 teams make the playoffs in 2006-2007 and in 2005-2006 Toronto had 90 points (2 away from a playoff spot), which would have made it 4 that year as well.
So, I decided to look into the chance of this actually happening. I have a script that simulates the whole season to do season predictions. I can randomize team skill or choose a certain skill level manually. A random distribution of skill produced a 63% chance of a 4 playoff team division and unbalancing one division jumped that number to 68%. I then decided to make every team identical (50% chance for every game) and that produced a even larger 69% chance. Either way there will be 4 teams who make the playoffs from one division 2 times out of 3 years based on my best analysis. In other words it's more common to have 4 teams make it from one division than not.