Season Four of Psych originally aired in the United States on USA Network from August 7, 2009, to March 10, 2010. It consisted of 16 episodes. James Roday, Dulé Hill, Timothy Omundson, Maggie Lawson, Corbin Bernsen, and Kirsten Nelson reprised their roles as the main characters. James Roday portrayed Shawn Spencer, a fake psychic detective who periodically consults for the Santa Barbara Police Department. A DVD of the season was released on July 13, 2010.
Steve Franks, the creator of the series, returned for the fourth season as show runner. The theme song "I Know, You Know" by The Friendly Indians continued to be used, though it was edited twice: in "Bollywood Homicide", the song was given a Bollywood theme with the lyrics sung in Hindi, while Boyz II Men performed an a cappella version for "High Top Fade-Out".
Mel Damski returned to direct four episodes, while Stephen Surjik returned for three and Steve Franks helmed two. Returning to direct one episode each were John Badham, Jay Chandrasekhar, Michael McMurray, James Roday, and Matt Shakman. Andrew Bernstein and Tawnia McKiernan each joined the show to direct one episode.
Steve Franks wrote five episodes, while Andy Berman wrote four and Kell Cahoon, Saladin K. Patterson, and James Roday wrote three. Returning for two episodes each were writers Tim Meltreger and Anupam Nigam. Bill Callahan and Todd Harthan joined the writing team, with Callahan writing three episodes, and Harthan writing one.
James Roday continued to play fake psychic detective Shawn Spencer. Burton "Gus" Guster returned, portrayed by Dulé Hill. Timothy Omundson returned as Head Detective Carlton "Lassie" Lassiter, while Maggie Lawson continued to portray Juliet "Jules" O'Hara. Corbin Bernsen was kept on as Henry Spencer. Kirsten Nelson continued in her role as SBPD Chief Karen Vick.
Sage Brocklebank continued to portray Officer Buzz McNab. Liam James and Carlos McCullers II continued in their roles as young Shawn and Gus, respectively. Rachael Leigh Cook appeared in four episodes as Abigail Lytar. Kurt Fuller made his Psych debut as Woody the Coroner, who shares many character traits with Shawn, in three episodes. Ally Sheedy made her second appearance as Mr. Yang, becoming the first villain on the show to appear in more than one episode. Jimmi Simpson made his final appearance as Mary Lightly, and Chris Turner entered as the chaotic, mysterious Mr. Yin, another deranged killer. Cary Elwes made his first appearance as art thief Pierre Despereaux, and Peter Oldring and Ed Lauter were seen as Canadian officers. Ray Wise made his first appearance as Father Westley. Jaleel White made his first appearance as Gus's former band mate. Additionally, almost every episode featured a prominent guest star. Among them were Christine Baranski, Jim Beaver, Josh Braaten, James Brolin, John Cena, Jay Chandrasekhar, Tim Conlon, Bruce Davison, Cullen Douglas, Deanna Dunagan, Miguel Ferrer, Azita Ghanizada, Ernie Grunwald, John Hawkes, Sandra Hess, Michael Hogan, Scott Holroyd, Steve Howey, Stacy Keibler, Alexandra Krosney, Stephan Lang, Joshua Malina, David Naughton, Judd Nelson, Larisa Oleynik, Robert Patrick, Sendhil Ramamurthy, Lisa Ray, Michael Rooker, Jeri Ryan, Chris Sarandon, Sarah Shahi, Craig Sheffer, Kenan Thompson, Tony Todd, Beverley Turner, Arnold Vosloo, and Thomas F. Wilson.
|Extradition: British Columbia||August 7, 2009||48|
|He Dead||August 14, 2009||49|
|High Noon-ish||August 21, 2009||50|
When strange things start happening at a simulated Old West tourist attraction that threaten to shut it down permanently, Lassiter enlists Shawn and Gus's help to uncover the truth and keep a part of his childhood open.
|The Devil Is in the Details... and the Upstairs Bedroom||August 28, 2009||51|
|Shawn Gets the Yips||September 11, 2009||52|
After a cop bar is shot up in an apparent robbery, Shawn realizes the shooter was actually targeting a member of their softball team.
|Bollywood Homicide||September 18, 2009||53|
|High Top Fade-Out||September 25, 2009||54|
Gus must team back up with his estranged college singing buddies when a member of their old quartet is murdered.
|Let's Get Hairy||October 9, 2009||55|
|Shawn Takes a Shot in the Dark||October 16, 2009||56|
Shawn's life hangs in the balance when, while working on a case involving an ice cream truck, he stumbles onto something bigger and far more deadly.
|You Can't Handle This Episode||January 27, 2010||57|
When a beloved local sea lion is found dead after being released into the wild, Shawn sets out to prove it was murder and stumbles onto a bigger conspiracy.
|Thrill Seekers and Hell Raisers||February 3, 2010||58|
|A Very Juliet Episode||February 10, 2010||59|
|Death Is in the Air||February 17, 2010||60|
|Think Tank||February 24, 2010||61|
Shawn and Gus are recruited into a think tank to prevent the murder of a business tycoon, but afterwards they realize that one of the members plans to use the assassination ideas they brainstormed to kill the client instead.
|The Head, the Tail, the Whole Damn Episode||March 3, 2010||62|
|Mr. Yin Presents...||March 10, 2010||63|
A killer targets Shawn using scenarios from classic Hitchcock films.