Season 4 - warning *SPOILERS*

Main Cast
The fourth season features six actors who receive star billing:
Emily Deschanel - Forensic anthropologist Dr. Temperance "Bones" Brennan.
David Boreanaz - FBI Special Agent Seeley Booth, who is the Official FBI Liason with the Jeffersonian.
Michaela Conlin - Forensic artist Angela Montenegro.
T. J. Thyne - Entomologist Dr. Jack Hodgins.
Tamara Taylor - Forensic Pathologist and the head of the Forensic Division Dr. Camille Saroyan.
John Francis Daley - Dr. Lance Sweets, a Psychologist who studies the relationship between Dr. Brennan and Seeley Booth.

Recurring Cast
Recurring characters in the fourth season include:
Eric Millegan as Dr. Zack Addy, who is now institutionalized.
Patricia Belcher as prosecutor Caroline Julian.
Ryan O'Neal as Max Brennan, the father of Temperance and Russ, who is now a free man.
Loren Dean as Russ Brennan, Temperance's older brother, who is on probation.
Ty Panitz as Parker Booth, Seeley Booth's son.
Nichole Hiltz as Roxie Lyon, a woman who was previously romantically involved with Angela.
Brendan Fehr as Jared Booth, Seeley's younger brother.
Marisa Coughlan as FBI Special Agent Payton Perotta, who fills in for Agent Booth when he was unavailable.
This season also introduces several new interns who fill Zack's position with each new case. The interns are:
Eugene Byrd - Dr. Clark Edison.
Carla Gallo - Daisy Wick.
Ryan Cartwright - Vincent Nigel-Murray.
Michael Grant Terry - Wendell Bray.
Joel Moore - Colin Fisher.
Pej Vahdat - Arastoo Vaziri.

Special Guest Cast
Scott Starret - Michael Badalucco, intern in "The Finger in the Nest".
Stephen Fry - Dr. Gordon Wyatt in "Mayhem on a Cross".
Billy Gibbons - playing himself as Angela's father, in "The Science in the Physicist".

The season premieres with its first two-part episode, "Yanks in the U.K." which was filmed on-location in the United Kingdom. This season sees the return of The Grave Digger, who was last seen in the second season. This time, The Grave Digger captures Booth. After Booth is rescued, the team discovers the identity of The Grave Digger, being Agent Heather Taffet, who was working on the case. The season also features multiple rotating interns who fill Zack's position with each new case.

4.01 & 4.02 - "Yanks in the U.K."
Directed by: Ian Toynton
Writer/s: Hart Hanson & Karine Rosenthal (Part 1)/ Stephen Nathan & Scott Williams (Part 2)

This two-hour season premiere, begins when Booth and Brennan in London, England, for a conference. They are then called upon to help solve a case. Meanwhile, Angela's husband, Grayson Barasa, is finally found, but he refuses to sign the divorce papers, much to the dismay of Hodgins and Angela.

4.03 - "Man in the Outhouse"
Directed by: Steven DePaul
Writer/s: Carla Kettner & Mark Lisson

After an explosion in an outhouse, a body is discovered to have been left in the sewage. The body turns out to be a reality TV host with a long list of people that might want him dead and Booth and Brennan are left to narrow down the suspects.

4.04 - "The Finger in the Nest"
Directed by: Jeff Woolnough
Writer/s: Lyla Oliver

Parker, Booth's son, discovers a corpse's finger in a bird's nest, causing the team to launch an investigation to try to find the rest of the body. After finding the corpse, the team discovers that the death was caused by a dog, leading them to an illegal dog fighting ring and a search for the killer dog and its owner. Cesar Millan makes a guest appearance.

4.05 - "The Perfect Pieces in the Purple Pond"
Directed by: Jeannot Szwarc
Writer/s: Josh Berman
The pieces of a dismembered body are found lying in a purple pool of water, except the head is missing. The team finds that the body belongs to Jared Addison, a science fiction writer who had several mental problems. With Sweets' help, several suspects are hunted down, however the team has trouble finding enough evidence with the victim's head still missing... that is until they turn to Zack for help.

4.06 - "The Crank in the Shaft"
Directed by: Steven DePaul
Writer/s: Elizabeth Benjamin
The remains of a despised office manager are discovered in an elevator shaft, and Booth and Brennan must figure out which employee killed her. After some investigating, they find that the manager had more than a fair share of complaints on file. Meanwhile, Angela and Hodgins try to make peace in the lab after their break-up.

4.07 - "The He in the She"
Directed by: Craig Ross, Jr.
Writer/s: Karina Csolty
When skeletal remains are found in the Chesapeake Bay, Brennan and Booth are on the case to investigate. With only the upper torso of the victim intact, the team has little to work from besides the remnants of the victim's breast implants. Brennan and Booth are led to a small church community in Maryland where the victim was a pastor, but they have no record of her existence prior to moving to the small town five years prior.

4.08 - "The Skull in the Sculpture"
Directed by: Allan Kroeker
Writer/s: Janet Lin
A corpse is found inside a crushed car like those used in car sculptures made by an artist named Geoffrey Thorne. Evidence leads to the art gallery where Thorne displays his work. Roxie Lyon arrives on the scene and presents herself as Thorne's assistant. Roxie states that Geoffrey has been depressed and that he's been talking about being part of his exhibit. She is one of Angela’s old friends.

4.09 - "The Con Man in the Meth Lab"
Directed by: Allison Liddi Brown
Writer/s: Karine Rosenthal
Booth and Brennan investigate the scene where a body was uncovered during a police training exercise. The team identifies the body as a struggling inventor's father who had recently reunited with his son. But when a second body surfaces, the team is led on a trail of deception to find out who was the actual con-man in the inventor's life. Meanwhile, Booth's younger brother Jared (guest star Brendan Fehr) moves to D.C. to take a job at the Pentagon.

4.10 - "The Passenger in the Oven"
Directed by: Steven DePaul
Writer/s: Carla Kettner
Booth and Brennan are en-route to China where Brennan has been summoned to help identify pre-historic anthropological remains with Booth in tow to watch over her highly-sensitive equipment. However, their flight is disrupted when a flight attendant discovers a fully-cooked human body in the plane's industrial microwave. When they identify the victim as a travel writer working on a controversial story about airline pilots, Booth and Brennan must race against the clock and solve the murder before the plane lands in China and they lose their jurisdiction over the case.

4.11 - "The Bone That Blew"
Directed by: Jessica Landaw
Writer/s: Carla Kettner
When a skull and various skeletal remains are found in a tree in a nationally protected wooded preserve, Brennan and Booth are on the case to investigate. Back at the Jeffersonian, Brennan learns of her father's new position as a teacher, a prospect she is less than happy to learn about, allegedly because of his recent acquittal for murder.

4.12 - "Double Trouble in the Panhandle"
Directed by: Dwight H. Little
Writer/s: Lyla Oliver
The team investigates the death of female conjoined twins who were in the process of deciding to get separated or not. The twins were working in a circus, which means that Booth will come face to face with a clown during the investigation. Brennan and Booth go undercover as Wanda and Buck Moosejaw, Canadian performers in a knife-throwing act. Sweets helps Brennan and Booth understand Circus lingo.

4.13 - "Fire in the Ice"
Directed by: Chad Lowe
Writer/s: Scott Williams
The body of a fireman is discovered in a lake and Booth is one of the suspects. The victim and Booth played hockey together. The investigation is conducted by FBI Agent Payton Perotta. During interrogation, sparks fly between Perotta and Booth. Brennan tries figure skating. NHL legend Luc Robitaille guest stars.

4.14 - "The Hero in the Hold"
Directed by: Ian Toynton
Writer/s: Janet Lin & Karine
The evidence from the Gravedigger case has gone missing, and Brennan and Hodgins, who has not fully recovered from being buried alive, are suspected of taking it. On the night of a ceremony recognizing Brennan, Booth is kidnapped by the Gravedigger. The team works on locating Booth and finding who is behind the kidnapping, with the help of Booth's brother Jared. Agent Perrota also appears.

4.15 - "The Princess and the Pear"
Directed by: Steven DePaul
Writer/s: Matthew Donlan & Jeremy Martin
With Booth in hospital with a bad back, Brennan must work with Agent Payton Perrota to investigate the murder of a promotional model for ImagiCon (a parody of the comic book convention Comic-Con). Their investigation leads them to discover that the murder of the woman might be linked to the possession of an age-old prop of an Excalibur from a famous French movie, La Mort d'Arthur.

4.16 - "The Bones That Foam"
Directed by: David Boreanaz
Writer/s: Elizabeth Benjamin
A bungee-jumping couple plunged off of a hot-air balloon after getting married only to come face-to-face with a corpse lying at the bottom of the gorge. After ruling out suicide, Brennan, Booth and the Jeffersonian team try to determine the cause of death. When they notice an unknown substance oozing from the body, the lab is thrown into immediate lock down, and they realize they have only 24 hours to unravel the mystery before the bones disintegrate.

4.17 - "The Salt in the Wounds"
Directed by: Steven DePaul
Writer/s: Carla Kettner & Josh Berman
Booth and Brennan investigate a petrified human body found buried in a mound of salt in a city de-icing truck. They discover that the victim was a pregnant 16-year-old high school volleyball player, and are shocked to find that nearly half of her teammates are either pregnant or already have had babies from the same father. Meanwhile, Roxie breaks up with Angela, who, seeking solace, has a one-night stand with Hodgins.

4.18 - "The Doctor in the Den"
Directed by: Ian Toynton
Writer/s: Janet Lin & Karine Rosenthal
The half-eaten body of Cam's former fiance is found in the tiger enclosure at the wild-life park, and Booth and Brennan determine the death was no accident. The team pieces together the fiance's final evening and questions suspects. Angela takes a vow of celibacy and we learn that Clark is romantically involved. In the end, Cam offers to adopt the doctor's daughter.

4.19 - "The Science in the Physicist"
Directed by: Brad Turner
Writer/s: Karina Csolty
Booth and Brennan are sent to investigate when human remains are found during a photo shoot. When the team uncovers a meteorite in the victim's ear, they are led to the Collar Institute of Science in D.C. There they learn the victim had been a part of a controversial project, Brennan and Booth must investigate the several death threats she had received. Meanwhile, Angela's father arrives in town to confront Hodgins.

4.20 - "Cinderella in the Cardboard"
Directed by: Steven DePaul
Writer/s: Carla Kettner & Josh Berman
When an image of the Virgin Mary turns out to be a bloodstain at a cardboard recycling plant Booth and Bones find a flattened body inside a compressed bale of cardboard. The line of suspects grows longer when the team discovers that the marriage-obsessed young woman had a fiance as well as numerous boyfriends from a cell-phone dating service. Angela and Hodgins come to terms with their relationship.

4.21 - "Mayhem on a Cross"
Directed by: Jeff Woolnough
Writer/s: Dean Lopata
When a skeleton is found being used as a stage prop for a Black metal band in Norway, authorities call the FBI upon discovering that the man, a bassist with the stage name Mayhem, is a US citizen. The investigation into underground extreme metal bands brings to light the teams' own surprising musical preferences and new information about Sweets' past. Meanwhile, Dr. Gordon Wyatt is back, helping Sweets write his book on Booth and Brennan by giving him a perspective on their relationship.

4.22 - "The Double Death of the Dearly Departed"
Directed by: Milan Cheylov
Writer/s: Craig Silverstein
When a colleague from the Egyptology department dies of heart failure, Brennan realizes at the wake that the death was not an accident. Brennan convinces Booth to steal the body so she and Cam can investigate. The true cause of death is baffling, and the suspects are as confusing as the death itself. Cam runs into trouble when she attempts to get Michelle, her newly adopted teenager, to stop smoking.

4.23 - "The Girl in the Mask"
Directed by: Ian Toynton
Writer/s: Michael Peterson
When the head of Sachi Nakamura, the sister of Booth's friend, is discovered in Washington, D.C., her brother Ken, a detective from Tokyo, flies in to aid the search for his sister's killer. The investigation leads to an elite escort service, and the team must race against the clock when Sachi's roommate is missing as well.

4.24 - "The Beaver in the Otter"
Directed by: Brad Turner
Writer/s: Scott Williams
Jimmy 'Beaver' Bouvier is found strung up in the costume of his college mascot, an Otter. Booth and Bones must investigate whether he was dead beforehand or killed during a Middlesex University bonfire event.

4.25 - "The Critic in the Cabernet"
Directed by: Kevin Hooks
Writer/s: Stephen Nathan
The team is on the case when human remains are found inside a barrel of wine during a wine tasting. When the victim turns out to be a wine critic whose scathing reviews were capable of sending a vineyard to its ruin, everyone from the winery's owner to the victim's own wife becomes a suspect. Meanwhile, Brennan decides to have a baby and asks Booth to be the father.

4.26 - "The End in the Beginning"
Directed by: Ian Toynton
Writer/s: Hart Hanson
All of the regular characters are seen living different lives. Booth and Brennan are married and own a nightclub called "The Lab", but when a body is found at the club, Detectives Cam Saroyan and Jared Booth are called to investigate. However, not everything is as it seems. Motley Crue guest stars in this episode as themselves.

Info from Wikipedia