Season Four begins with Dean awakening dazed and confused in a grave, having been rescued by an angel named Castiel on orders from God. Sam has further developed his abilities with the help of Ruby during the months that Dean has been dead. It is soon revealed that Dean was revived to help stop Lilith's plan of breaking the 66 seals, which would let Lucifer walk free once again. In the season finale Sam kills Lilith, but her murder is the final seal. The angels also wanted the seals to be broken, because when Dean defeats Lucifer, they can usher in paradise on Earth. Ruby is revealed to be manipulating Sam since her first episode, and Dean kills her in revenge. With Sam having killed Lilith, and ultimately breaking the final seal, Lucifer is now free.

- WIKIPEDIA

Cast:

The fourth season features two major roles with star billing. The show continues to chronicle the lives of two brothers who travel across the United States fighting against demons. Jensen Ackles stars as demon hunter Dean Winchester. Jared Padalecki portrays Sam Winchester, brother to Dean and fellow demon hunter.
The show always features guest stars. Jim Beaver returns as Bobby Singer, a fellow hunter who acts as a mentor/surrogate father to the Winchester brothers. Genevieve Cortese portrays demon Ruby, who is possessing a new human body following season three cast member Katie Cassidy's departure from the show.
Nicki Aycox who portrayed demon Meg Masters in the first season, returns to portray the ghost of the human Meg Masters, whose body had been possessed by the demon.
Also returning to the show for brief appearances are: FBI agent Victor Henricksen, Ronald Reznick, John Winchester, Mary Winchester, and demons Lilith and Azazel. Keegan Connor Tracy also returned to the show to portray publisher Sera Siege in The Monster at the End of this Book, although this is a different role than she played in Season 2, when she had the part of Karen Giles in The Usual Suspects.
New characters include:
Castiel, Pamela, Uriel, Anna, Alastair, and Zachariah.