The fourth season of Monk originally aired in the United States on USA Network from July 8, 2005, to March 17, 2006. It consisted of 16 episodes. Tony Shalhoub, Traylor Howard, Ted Levine, and Jason Gray-Stanford reprised their roles as the main characters. A DVD of the season was released on June 27, 2006.
A jewelry store manager goes missing and a security guard is killed in an apparent robbery. When small-time private detective Marty Eels (Jason Alexander) shows up at the scene to offer a series of remarkably accurate insights – including the whereabouts of the manager – Monk becomes determined to expose him as a fraud. Also guest stars Dana Ivey.
While investigating the murder of an armored car driver in a supermarket parking lot just before Halloween, Monk gets a call from Ambrose (John Turturro) telling him that their long-lost father is coming home. As the brothers, Natalie, and Julie spend an uneasy night waiting for him, they run across a trick-or-treater whose habit of stealing candy intrigues Monk.
Natalie finds a pizza delivery man beaten to death in his own car, but the investigation into his murder is sidetracked by the disappearance of a superior court judge. Natalie probes the first case as the police work on the second, with Monk increasingly drawn into both of them as he tries to recover from the flu at home.
Monk looks into a double attack at a parking garage that has left an attendant dead and a stock analyst's hand broken. As Monk goes undercover at the analyst's firm to investigate the case, he finds himself dealing with suspicions of insider trading, an inter-office bowling rivalry, the murder of an interior decorator, and the fact that his coworkers actually like him. Guest stars Eddie McClintock.
While Monk and Natalie are visiting a winery for the weekend to commemorate his honeymoon with Trudy, he makes the acquaintance of a man (Daniel Roebuck) who seems to disappear from the grounds overnight. None of the guests and staff claim to have seen him, and Monk becomes determined to uncover the truth when a contract killer (Paul Ben-Victor) arrives looking for the man. Also guest stars Felicia Day, Chris D'Elia, and Richard Libertini.
As Monk starts to make progress in dealing with the death of his wife Trudy (Melora Hardin), Natalie encounters a woman who looks exactly like her. The evidence mounts that Trudy may have faked her murder and been involved in the death of an old man (Harve Presnell), whose daughter worked with her on a news story that sent a union boss to prison. Also stars Kevin Kilner and Jarrad Paul as Kevin Dorfman.
Accompanying Natalie to the wedding of her brother Jonathan (Rob Benedict), Disher is struck by a car in the parking lot of the hotel where it is being held. While Natalie tries to reconcile with her parents (Michael Cavanaugh and Holland Taylor), Monk and Stottlemeyer struggle to figure out if someone in the wedding party is responsible for both this attack and the murder of the event photographer, found dead in a spa mud bath. Also stars Ashley Williams.
One of Monk's eighth-grade classmates, on whom he had a crush, comes to him for help in solving a break-in that resulted in the death of her housekeeper. As Monk tries to win her affections and searches for clues to explain a defaced painting found at the crime scene, he remembers his efforts to clear her name in a theft of school bake sale money over 30 years earlier. Grant Rosenmeyer plays Monk's younger self.
During the SFPD Christmas party, a detective dies after drinking from a poisoned bottle of port intended for Stottlemeyer. The captain recruits Monk and Natalie to determine whether the brother of a bank robber he killed is out for revenge, and Monk has to go undercover as a shopping mall Santa in order to get to the truth. Guest stars Rachael Harris.
A garment factory worker begs Monk to prove the innocence of her son, who is in jail awaiting trial for the murder of a fashion model. The evidence against the young man is overwhelming, but Monk begins to question it as he investigates a short-tempered designer (Malcolm McDowell) and the victim's roommate (Mini Andén), the latter of whom soon turns up dead as well. Also guest stars Scott Adsit.
While pursuing a lead on Trudy's murder, Monk is robbed, knocked unconscious, and dumped on a truck heading out of San Francisco. Waking up with amnesia in a small Wyoming town, he is taken in by a lonely woman (Laurie Metcalf) who claims to be his wife and finds himself unable to resist investigating the sudden disappearance of a diner waitress. Also guest stars Charles Napier and Michael Shalhoub.
Stottlemeyer's worry that his marriage to Karen is falling apart threatens to derail the SFPD's investigation into the murder of a drug dealer. The captain is sent to anger management class after punching an officer who claims to have slept with her, leaving Disher, Monk, and Natalie to work the case with help from a homeless man who witnessed the crime.
Final appearance of Glenne Headly as Karen Stottlemeyer.
When a priceless diamond is stolen from a museum, Monk and Natalie find themselves competing against three private detectives (DJ Qualls, Derrick O'Connor, and Tyler Mane) to find it and claim a $1 million reward. As the case leads them to a meditation center where they discover a prime suspect dead, Disher finds himself dealing with a woman who repeatedly comes to the precinct to confess to trivial crimes.
A former showgirl is found dead of an apparent suicide shortly before her tell-all memoir is to be published, but Monk suspects one of her past lovers, a famous astronaut (Jeffrey Donovan), of murdering her. To prove it, though, he must break the man's unassailable alibi that he was orbiting the Earth at the time, and also figure out a connection to a mail-stealing neighbor whose garage door opener has gone haywire.
While under sedation during a dental appointment, Disher thinks he sees the dentist (Jon Favreau) and his assistant (Brooke Langton) kill an intruder in an argument over Barry Bonds. He quits the SFPD in frustration after the intruder turns up dead and no one takes him seriously, but Monk and Stottlemeyer find evidence linking the man to an armored car robbery and double homicide and start to believe Disher's claim.
While Monk is serving on a jury during a robbery trial, the SFPD must handle the extradition of a fugitive wanted on federal drug trafficking charges. Monk's suspicion of a body being hidden in a dumpster outside the courthouse is proven correct, and he begins to connect this murder to both his trial and the fugitive's impending handover to federal authorities. Guest stars Emmanuelle Vaugier and Wings Hauser.
