Season 4 List of Episodes

Episode 24 - The Roomate Transmogrification
When Bernadette receives her Ph.D., the guys take joy in reminding an emasculated Wolowitz that he is the only one in their group who isn't a doctor. Meanwhile, Koothrappali becomes Sheldon's new roommate after hearing Leonard and Priya engaging in a "Star Trek" bedroom fantasy.

Episode 23 - The Engagement Reaction
When Howard finally breaks the news to his mom that he's engaged to Bernadette, it lands her in the hospital.

Episode 22 - The Wildebeest Implementation
Raj attempts to cure his social anxiety disorder while Penny uses Bernadette to spy on Leonard and his girlfriend.

Episode 21 - The Agreement Dissection
When Priya uses her lawyer skills to pick apart the roommate agreement, the girls take Sheldon dancing.

Episode 20 - The Herb Garden Germination
Sheldon and Amy experiment on their friends by spreading gossip, while Howard takes a big step in his relationship with Bernadette.

Episode 19 - The Zarnecki Incursion
When someone hacks Sheldon's online game account, the guys go on a quest to find the culprit.

Episode 18 - The Prestidigitation Approximation
Leonard must choose between new girlfriend Priya and his friendship with Penny. Meanwhile, Howard drives Sheldon crazy with a magic trick.

Episode 17 - The Toast Derivation
Realizing Leonard is the center of the gang, Sheldon seeks to form a new group of friends of which he can be the center.

Episode 16 - The Cohabitation Formulation
Wolowitz finds himself having to choose between Bernadette and his mother. Leonard has another rendezvous with Raj's sister Priya.

Episode 15 - The Benefactor Factor
A wealthy donor to the university makes Leonard consider how far he's willing to go for the sake of science.

Episode 14 - The Thespian Catalyst
Hoping it will make him a better teacher, Sheldon has Penny give him acting lessons. Raj has fantasies about Bernadette.

Episode 13 - The Love Car Displacement
A science convention at a hotel leads to Bernadette running into an ex-boyfriend and Wolowitz worrying he can't quite compare.

Episode 12 - The Bus Pants Utilization
Leonard's idea for a smartphone app derails his friendship with Sheldon.

Episode 11 - The Justice League Recombination
The guys find a "super" use for Penny's new boyfriend, Zack, when they enter a costume contest as the Justice League.

Episode 10 - The Alien Parasite Hypothesis
Amy crushes on one of Penny's dim ex-boyfriends. Raj and Wolowitz argue over who'd be the better superhero.

Episode 9 - The Boyfriend Complexity
Leonard is flattered when he shows up at Penny's door and gets a kiss from Penny. Later, Penny informs him that her critical father Wyatt (Keith Carradine) is visiting, and she asks him to pretend that they're back together because he's the only boyfriend she's ever had that he likes.

Episode 8 - The 21-second Excitation
Sheldon, Leonard, Raj, and Howard excitedly plan to attend a screening of Raiders of the Lost Ark that includes 21 seconds of never-before-seen footage. However, they delay reaching the theater, resulting in a lot of complaining from Sheldon. After a short time, Wil Wheaton arrives for the show. Much to Sheldon's dismay, Wheaton is ushered into the theater ahead of everyone, and the four are subsequently denied access to the theater because of this.

Episode 7 - The Apology Insufficiency
Howard is trying to get top-level security clearance, and in order to clear him, the attractive G-man will interview his friends about his past and any secrets he may have. However, Sheldon accidentally talks about when Howard got a Mars rover stuck in a ditch in The Lizard-Spock Expansion from season 2, and Howard is subsequently denied clearance off-screen. After unsuccessfully attempting to retract his statement, Sheldon apologizes to Howard, but Howard does not accept his apology.

Episode 6 - The Irish Pub Formulation
Raj's sister Priya (Aarti Mann) visits and inadvertently causes trouble for the friends. She and Leonard apparently have a history, despite Leonard's pinky swear with Howard that they would not interact with her in deference to Raj. They hook up, but are discovered by Sheldon, who tries to devise an elaborate, flawless lie to cover it up. Leonard cannot persist in the deception and reveals the assignation. The guys then confess to all of the deceptions they have perpetrated on each other, including Sheldon's experiment of feeding moth parts to Leonard. The episode ends with Sheldon preparing snow cones made with mango caterpillar, which grosses Leonard out.

Episode 5 - The Desperation Emanation
When Amy Farrah Fowler asks Sheldon to meet her mother, it worries Sheldon. He still adamantly says that Amy is his friend but not his "girlfriend". When Leonard reveals to him that he can't actually say that anymore, it freaks him out and he tries to "break up" with her. When she tells him it is only to please her mother, and that she doesn't actually want to be his girlfriend, he is happy again. He goes along with her idea and tells Amy's mother via webcam that he is having regular sex with her daughter, which freaks her out just like Amy wanted it to. Sheldon then asks Amy if they actually want to have sex, and then says "Bazinga!" She leaves and he goes to bed.

Episode 4 - The Hot Troll Deviation
When the guys run into Howard's ex at the Cheesecake Factory, he ends up confronting his lingering feelings for her. He seeks help from Penny, who agrees on the condition that Howard reveal the reason behind his breakup. He confesses that Bernadette caught him engaging in cybersex with a fellow World of Warcraft player, later revealed to be a male colleague at the university. Howard and Bernadette discuss their relationship at the restaurant and make amends, deciding to give it another try. They go out on a date and are last seen making out in her car.

Episode 3 - The Zazzy Substitution
Sheldon and Amy Farrah Fowler are very compatible with each other, as evidenced by how well they can play "counter-factuals", a game they invented requiring them to know what the other is thinking would be the answer to a question about "an alternate world that differs from ours in one key aspect" (Leonard flunks miserably when he attempts to play this game). Bringing Amy into the group is a source of consternation, as the guys can barely handle Sheldon by himself, let alone with Amy who is his female equivalent.

Episode 2 - The Cruciferous Vegetable Amplification
Sheldon's estimate for his life expectancy leads him to the conclusion that he will miss by only a few years "the singularity ... when man will be able to transfer his consciousness into machines, and achieve immortality". He therefore changes his diet to eat cruciferous vegetables and starts to exercise in hopes of extending his life expectancy. Since the diet of brussels sprout gives him a bad case of flatulence, and he tumbles on the stairs going down for a jog, Sheldon decides on an alternate plan to extend his life expectancy.

Episode 1 - The Robotic Manipulation
The episode starts with the five main characters having dinner and then oscillates between two plots. In one plot, Howard demonstrates during dinner the capabilities of a robotic arm, that he "borrowed" from the Jet Propulsion Laboratory by showing off how the robotic arm can serve dinner. He later uses the robotic arm to give himself a massage. Since the arm feels so real, it gives him the idea to use the robotic arm to masturbate.