In the third season, the plot of the show focused on trying to save Dean from the deal he made with Azazel, in which he sold his soul in order to resurrect Sam.
Along the way they met a demon named Ruby, who has an interest in Sam and claims to be able to help save Dean, as well as Bela Talbot, an "acquirer" and seller of occult objects who is constantly a thorn in their side.
The brothers eventually managed to track down the demon who holds Dean's contract, a powerhouse named Lilith who also wants Sam dead. However, Sam and Ruby ultimately failed to save Dean, who was killed and sent to Hell.
Season 4 begins with Dean awakening dazed and confused in a grave, having been rescued by an angel named Castiel on orders from God.
Sam on the other hand, has further developed his abilities with the help of Ruby during the months that Dean has been dead.
Dean has a joyful reunion with Sam and Bobby, but it isn't long before the three wonder exactly what pulled Dean from Hell and what it will want in return.