A fantastic series to watch, and one of the better series available on amazon prime.
4 episodes, each 1.5+ hours long, each episode representing a season--winter, spring, summer, etc. Each episode focuses on one murder mystery, but continuity is established by the detectives' personal lives and the mystery surrounding Commander Rousseau's missing son, which wraps up at the end of the 4th episode.
The only complaint I have is there is only 1 season available to watch on Prime.