The probability of observing 4 blocks in a minute when those come from Poisson distribution (the probability of a given number of events occurring in a fixed interval of time and/or space if these events occur with a known average rate and independently of the time since the last event - Wikipedia) is P(X=4) = 0.01532 (calculation here: http://www.wolframalpha.com/input/?i=Poisson+distribution+X%3D4+mean%3D1). This makes it happen about 22 times a day which is once each 65 minutes on average. When the difficulty changes those numbers could slightly vary. We don't have any statistical evidence about something fishy occurring.