Supernatural Season 4 premiered on September 18th, 2008 and concluded on May 14th, 2009. It consisted of 22 episodes beginning with the episode "Lazarus Rising" and ending with the episode "Lucifer Rising".
It begins with Dean miraculously coming back from Hell. After searching down Sam and Bobby, they then find out that Dean had been resurrected by Castiel, an angel, because they need him for some unknown purpose. It is later revealed that this purpose is to stop the Apocalypse. Sam has also reunited with Ruby the demon and has learned how to exorcise demons using only his mind. after learning the certain omens that have started are signs of the coming apocalypse, the season's mission is to stop Lucifer from rising from Hell. Sam believes that killing Lilith, the "first" demon, will stop the rising, but it is revealed in the finale, that killing her is the final seal of 66 that need to be broken to acomplish the rising of Lucifer. As the season progresses, Castiel becomes more sympathatic to humankind. However, him and Dean are unable to stop Sam before it is too late, and Lucifer is risen to walk the earth.