Oh man, we are in a pause mid episode and it's all happening, the cliff hanger for season 5 is set. McNaulty is upset over Boodies death, Freeman is more or less leading the major crimes investigation, Daniels is being groomed for a substantial promotion.
I feel like the polarity between good cops and bad cops is exemplified well in season 4 with Carver and Haulk. They grew together as rookies through the western then into narcotics and major crimes until they saw different roads to better their positions. Conversely to their partnership the relationship between the roads they do take is definately one of this series themes, politics in police work.
Carver's return to the western shows him as an honest cop looking to get back to real police work, or at least the police work who grew up doing. While there we see him take a leadership position and also report to the notorious Bunny Colvin (one of my personal favourite characters).
Though he disappoints the viewer and his commanding officer on a couple of occasions we eventually see his epiphany and the fruition of proper police work.
Haulk on the other hand looks to climb the ladder, taking on a laid back assignment protecting the mayor with the notion that everyone else he knows to have taken the assignment has moved up. After his assignment is cut short by an unforeseen promotion he serves in major crimes as a sergeant(?) to the disappointment of all. No one is on Haulk's side.
Though they both kicked off the season getting their stripes the Gospel of the Baltimore Police Department chose only one to keep them.