Traveling throughout America, protagonists Sam (Jared Padalecki) and Dean Winchester (Jensen Ackles) use their father's journal to help them carry on the family business—saving people and hunting supernatural creatures.

As Dean's deadline approaches, their efforts are further hindered by Bela Talbot (Lauren Cohan), a professional thief of occult items who is often at odds with the Winchesters.

"[8] Budgetary reasons brought about the replacement of Cassidy for the fourth season,[9] while the character of Bela was removed due to the negative fan reaction.

Actor Jim Beaver returned as hunter Bobby Singer, and felt the character had grown into a surrogate father for Sam and Dean over time.

[11] Richard Speight Jr. returned as the Trickster in "Mystery Spot", as did Travis Wester and A. J. Buckley in "Ghostfacers" as Harry Spangler and Ed Zeddmore.

[13] The writers also considered bringing back Charles Malik Whitfield for a recurring role, with his character, FBI Agent Victor Henriksen, continues his hunt for the brothers throughout the season.

Sterling K. Brown made his final appearance as the vampire hunter Gordon Walker in "Fresh Blood" after a brief role in "Bad Day at Black Rock".

The character's story arc for the season was intended to be longer, but Brown's commitments to the Lifetime Television series Army Wives limited his return to only two episodes.

Sandra McCoy, who played a host to the Crossroads Demon in "Bedtimes Stories", began dating Padalecki after working with him on the 2005 film Cry Wolf.

Due to the time required to apply the extensive make-up and prosthetics for the role, Drago ended up with a minimum of 20-hour work days.

[25] Kripke noted that Dean's demonic deal of the previous season provided the writers with "a lot of effective emotional context to play with".

"[32] As time passed, Kripke felt that the terrorist-cell format was not working out as planned, partly because he prefers writing based on old legends rather than current topics.

The writers intended for him to save Dean from Hell, possibly even before the season finale, by giving into his demonic powers and becoming "this fully operational dark force" who would then want to go after Lilith.

[36] Despite this, Kripke felt the strike's effects ended up making the series "meaner, leaner, and more concise",[37] as they were able to focus the remaining episodes onto "the storyline [they] needed to pay off"—Dean's demonic pact.

The writers chose the deviate from folklore, making the changelings older in "The Kids Are Alright" to avoid having Sam and Dean blowtorching babies.

[6] The episode's sequence in which a character finds maggots in his hamburger was inspired by Kripke's "horrific" discovery of a maggot-covered possum in his garbage can.

[43] The episode "Sin City" was originally only meant to be written by Jeremy Carver, who pitched a concept similar to the film Enemy Mine—Dean would be trapped with a demon in a wine cellar.

[51] Both "Red Sky at Morning" and "Bedtime Stories" used Burnaby's Heritage Park; it functioned as a cemetery for the former, while the latter used it as the site of a giant gingerbread house.

Lisa's kiss with Dean at the end of "The Kids Are Alright" was unscripted; director Phil Sgriccia convinced actress Cindy Sampson to do so because he wanted to see how Ackles would react.

[65] The episode "Mystery Spot" heavily relied on visual effects for Dean's various death sequences by making use of a computer-generated 3D model of Ackles.

[66] For the changelings in "The Kids Are Alright", Kripke merely instructed Hayden to make the children's faces pale and have dark circles beneath their eyes.

[40] Though the script for "Red Sky at Morning" described the confrontation between the ghostly sailors as "they collide into a swirling vortex that disappears", Kripke and Singer ultimately left creative control up to Hayden.

[71] The spirit attacks Bela in the episode's climax, which made use of a contraption built by special effects makeup artist Tony Lindala; a tube connected to a denture on the off-camera side of Cohan pumped out large amounts of water, creating the illusion that she is vomiting it out.

[73] For example, Lennertz penned distinct music for each Sin in "The Magnificent Seven", with a "slow, lumbering, creepy low-end thing" theme for Sloth.

[60] Lennertz feels that "people associate the sound of violins with vampires" due to the "connection with Eastern Europe and counts", and used a "very violin-heavy" score for "Fresh Blood".

[81] Tim Janson of Mania felt the season moved "at a breakneck pace", describing the viewing experience as "being on a trail speeding headlong into the unknown".

Although she generally enjoys season-long story arcs, Steenbergen felt that Dean's time limit signified to viewers that the plotline would not be resolved until the season finale.

She found the season premiere to be "pretty boring", but called "Jus in Bello" to be "one of the best episodes of the year, maybe even of the show itself", because it begins with an "epic battle" setting but still "focuses on the personal level of the Winchesters and the people around them".

The "likeable secondary characters" of Charles Malik Whitfield's Agent Henrickson and Jim Beaver's Bobby Singer were welcomed back.

[83] While there were "a number of good episodes", Maureen Ryan of the Chicago Tribune pointed out the "few outright clunkers" such as "Red Sky at Morning".