Azazel traps the psychic children in a ghost town for a battle royale. Sam is stabbed and killed. A devastated Dean sells his soul to a Crossroads Demon to resurrect Sam, giving himself exactly one year to live. They kill Azazel, but not before a Hell Gate is opened, releasing hundreds of demons.
Widely considered a turning point in the series, Season 4 introduces Judeo-Christian mythology to the Supernatural universe. Supernatural all seasons 1- 2- 3- 4- 5- 6- 7- 8- 9