Season 4 fundamentally transformed the show. With Dean pulled out of Hell by the angel Castiel and Sam developing demon-blood-fueled psychic powers, the mythology expanded from a ghost-and-monster show into a cosmic drama about the Apocalypse.
Creatures Introduced
The massive introduction this season is angels — specifically Castiel and the archangels. The rugaru was introduced, along with the siren (adapted from Greek mythology), ghouls, and the Whore of Babylon. The demon hierarchy was expanded with the introduction of Alastair (Hell's chief torturer) and further development of Lilith as the primary antagonist.
Major Lore Developments
Angels were established as real, powerful, and not necessarily benevolent. The 66 Seals — mystical locks keeping Lucifer imprisoned — became the central plot device, with Lilith breaking seal after seal. Dean's time in Hell was explored, revealing that he had both been tortured by and eventually become a torturer under Alastair. Sam's demon blood addiction was revealed as a deliberate manipulation by Ruby to prepare him to break the final seal. The season ended with Sam killing Lilith — which was itself the final seal — and Lucifer rising from his cage.