The simplest way to answer this would probably be to look at how many the times final four were the previous four Stanley cup winners. Since the NHL has run the playoffs since 1926, one would start with by seeing how many times it has happened since 1930, divided by the total number of Stanley cup playoffs since 1930. According to the 2013 Wikipedia article, "For the first time since 1945, the final four teams left in the playoffs were the previous four Stanley Cup champions: Pittsburgh (2009), Chicago (2010), Boston (2011), and Los Angeles (2012)", so the probability will be very low.
It's a small sample size, of course, so it can only be a rough estimate of the probability. It's also going to be a terrible estimate because the number of teams has changed since 1930, and rules have changed, etc., so there's all sorts of problems with the calculation, but it's simple at least.
Less simple would be to come up with an actual statistical model of how likely each time is to beat any other given team in the NHL, use that to calculate the probability that they make it into the playoffs, the probability that they make it into the final four, the probability that they were the champs X years ago, etc, in a terrible Bayes' Law calculation.
Or maybe you can just pester Nate Silver until he does all that for you.