Not all of the episodes were high drama, thankfully. Some of the best comedy moments in the show's history were in season four and they were welcome respites from the scares and horror of the more typical monster of the week episodes. My favorites were the life-size teddy bear in "Wishful Thinking," the lederhosen scene in "Monster Movie" and nearly every moment of "The Monster at the End of This Book" including the introduction of the very entertaining Rob Benedict as Chuck. Jensen Ackles and Jared Padalecki have excellent comic timing and the more chances they have to explore this side of things the better.
While Supernatural may have started out as a type of X-Files-lite in its first season, it has increasingly proven that there is nothing light about the story they are telling. A solid foundation was built from the beginning, emotionally connecting the Winchester family to the monsters they hunt, and season four develops things further, taking elements of the backstory that the audience may have thought they knew and expanding them in new and surprising ways. This is best exemplified with Dean being sent back in time to meet his parents in the episode "In The Beginning." That story forever changed the way we view the Winchester family, but at the same time everything fits neatly into the previous backstory.
Jim Beaver as Bobby provided his usual outstanding backup to the main cast. His father figure has become an essential piece of the Winchester family. His reunion with Dean in "Lazarus Rising" is nearly as emotional as Sam's. Less successful was the casting of Genevieve Cortese as the new Ruby. She was acceptable for the most part, but when things got more serious in the end she seemed out of her depth in the acting department, it is sad to say.
While there were some ineffective moments in the season, overall season four of Supernatural more than achieved the goal of expanding the story into larger and more exciting directions. With the cliffhanger they left us at the end, I'm very much looking forward to where they are going to take us next season.