The fourth season 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.
Shawn and Gus spot an art thief whom Lassiter has been trying to arrest for years. So, although they are on a skiing vacation in Canada, they appeal to the local constabulary with mixed results: someone gets axed, another finds himself interrogated and Pierre Despereaux may get away scot-free, once again.
A billionaire hedge fund manager asks Shawn and Gus to find his murderer – a last request Shawn can't resist. So he and Gus step into a world of privilege that's rivaled only by the depths of betrayal it contains.
Lassiter reluctantly asks Shawn and Gus for help when his surrogate father, sheriff of Sonora, thinks a ghost is wreaking havoc in his town. And even though this Wild West re-creation town is bigger on show than facts, it turns out a real crime is being planned and the murder of the Sheriff's business rival is only the beginning.
When a college girl at a Catholic university commits suicide, Gus and Shawn are brought in because one of the professors at the school believes the girl was demon-possessed.
After a cop bar is shot up in an apparent robbery during Officer McNab's birthday celebration, Shawn realizes the shooter was actually targeting Lassiter.
Raj, a young East Indian man whose serious girlfriends have all met with suspicious accidents, is convinced that he is the victim of a curse, but Shawn and Gus aren't buying the supernatural explanation. They are convinced that a mysterious assailant is actually responsible and they're out to catch the culprit.
Shawn and Gus investigate the death of Leonald Callahan, aka Diddle, a computer cryptologist who was the baritone in Gus’s old college a capella group.
Shawn and Gus attempt to protect Stewart Grumbley from himself because the poor man believes he may be a sheep-killing werewolf who may soon enough begin killing humans.
While investigating a possible crime, Shawn stumbles upon one of a pair of criminals, gets shot, and is dumped in the trunk of a car.
Abigail is disappointed when Shawn accepts a case at the Army base instead of going on a date with her, but he and Gus are intrigued by a death they believe shouldn't have been ruled a suicide and the thought of working with Juliet's brother.
When Shawn and Gus join Gus’ new girlfriend on a rafting trip, a member of the group goes overboard, but Shawn quickly realizes it was no accident.
In 2003, Juliet made a pact with her boyfriend Scott Seaver (Josh Braaten) to meet up in seven years. After he fails to show up, Shawn finds him, and by extension endangers his life since he was a key witness against mobster J.T. Waring (Arnold Vosloo), who was convicted for the murder of a U.S. Marshall. When Waring denies killing the Marshall, Shawn must prove him right in order to protect Scott.
Shawn and Gus must recover a deadly pathogen before a thief releases it into the city.
Shawn and Gus are recruited into a think tank to help prevent the murder of a business tycoon, but once the group has disbanded, they realize that one of the members plans to use the assassination ideas to kill the client instead of save him.
After Lassiter takes a page out of Shawn's book and claims a shark attack victim was actually murdered, the guys must then scramble to find the fish and prove him right.