Season Four consists of 22 episodes that aired on Thursdays at 9:00 pm in the United States beginning September 18, 2008, and ending May 14, 2009.
Dean is rescued from hell and brought back by an angel named Castiel; one of several angels that appears throughout the season. The rest of the season follows the brothers as they work with the Castiel to stop Lilith's plan of breaking the 66 seals, which would allow Lucifer to walk free once again. Sam and Dean's relationship is also strained as Sam starts siding with Ruby over Dean. He also begins to give into his demonic side by drinking demon blood to become strong enough to defeat Lilith. After tracking down and killing Lilith, Sam learns that her death is actually the last seal and that Ruby was deceieving him all along. Dean arrives too late to stop him, but kills Ruby as Lucifer's prison opens up beneath them.