Season 4 of The Big Bang Theory returned to television on September 23rd, 2010 and contained 24 episodes. Throughout the season, both Sheldon and Howard become more involved in their girls/girlfriends and Penny realizes her true feelings for Leonard following their third season break-up, especially after Leonard begins to date Raj's younger sister, Priya.



Episode (Series) Episode (Season/Ep#) Title Original Air Date Directed By Description
64 401 The Robotic Manipulation September 23, 2010 Mark Cendrowski Howard demonstrates a robotic arm, that he "borrowed" from the Jet Propulsion Laboratory.  To everyone's surprise, Sheldon announces his plans to conceive a progeny with Amy via in vitro fertilization. Penny comes to know of Sheldon's new friend and asks him to take Amy on a date which she ends up chaperoning.
65 402 The Cruciferous Vegetable Amplification September 30, 2010 Mark Cendrowski Sheldon determines that his best estimate for his life expectancy by changing his diet and then building a robot to take his place.
66 403 The Zazzy Substitution October 7, 2010 Mark Cendrowski Sheldon and Amy Farrah Fowler are very compatible with each other. They break up over who's profession is more important which causes Sheldon to start collecting cats.
67 404 The Hot Troll Deviation October 14, 2010 Mark Cendrowski 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.
68 405 The Desperation Emanation October 21, 2010 Mark Cendrowski When Amy Farrah Fowler decides to introduce Sheldon to her mother, it worries Sheldon. He decides to end their platonic friendship.
69 406 The Irish Pub Formulation October 28, 2010 Mark Cendrowski Raj's sister Priya 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 devises an overly elaborate lie to cover it up.
70 407 The Apology Insufficiency November 4, 2010 Mark Cendrowski When Howard attempts to receive a high security clearance, his friends are interviewed by a formidable FBI agent checking into Howard's past. Raj and Leonard are pathetic during their respective interviews, while Sheldon reveals too much.
71 408 The 21-Second Excitation November 11, 2010 Mark Cendrowski 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 much complaining from Sheldon, which becomes worse when Wil Wheaton arrives. Amy starts to bond with Penny during a sleep over.
72 409 The Boyfriend Complexity November 18, 2010 Mark Cendrowski 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 is visiting, and she asks him to pretend that they're back together because he's the only boyfriend she's ever had that her father likes.
73 410 The Alien Parasite Hypothesis December 9, 2010 Mark Cendrowski Amy begins to have feelings for Penny's ex Zack, whom she meets in a bar on a night out with Penny. After she and Sheldon decide that Amy is horny, Sheldon sets her up on a date with Zack.
74 411 The Justice League Recombination December 16, 2010 Mark Cendrowski The guys discover that Penny and her ex-boyfriend, Zack, are back together. Much to Penny's annoyance, she and Zack end up going to the comic book store New Year's Eve party as Wonder Woman and Superman.
75 412 The Bus Pants Utilization January 6, 2011 Mark Cendrowski Sheldon, Howard, and Raj are eating together in the Cafeteria when Leonard approaches them with an idea for a smartphone app that helps users solve Differential Equations. Leonard quickly recruits Howard, Raj, and reluctantly Sheldon who becomes too obnoxious and gets bumped from the team.
76 413 The Love Car Displacement January 20, 2011 Anthony Rich Leonard, Sheldon, Howard, Raj, Amy, and Bernadette are invited to be on a discussion panel at  a symposium in San Francisco. Amy invites Penny as her guest which results in muliple hotel room bed hopping.
77 414 The Thespian Catalyst February 3, 2011 Mark Cendrowski Sheldon gives a lecture in a class and receives mulitple negative comments. To help him, Amy suggests that he take acting lessons from Penny.
78 415 The Benefactor Factor February 10, 2011 Mark Cendrowski President Siebert makes the gang attend a fundraiser for the university. One of the attendees invites Leonard to dinner.
79 416 The Cohabitation Formulation February 17, 2011 Mark Cendrowski Bernadette asks Howard to consider living together. Howard fights with his mother and moves out. Leonard and Priya start their relationship.
80 417 The Toast Derivation February 24, 2011 Mark Cendrowski Sheldon is upset when he realizes Leonard is the center of their group of friends. When the gang starts hanging out at Raj's, Sheldon decides, on advice of Amy and Penny, to get a new group of friends, which consists of Kripke, Stuart, Zack, and LeVar Burton.
81 418 The Prestidigitation Approximation March 10, 2011 Mark Cendrowski Priya talks Leonard into getting contact lenses. Howard and Raj perform a magic trick that Sheldon becomes obsessed with figuring out. 
82 419 The Zarnecki Incursion March 31, 2011 Peter Chakos Sheldon's World of Warcraft account was hacked and his character was raided clean, the boys are determined to track down this hacker. Penny helps them deal with the thief.
83 420 The Herb Garden Germination April 7, 2011 Mark Cendrowski Amy and Sheldon use the members of their social group for an experiment on the spread of gossip.
84 421 The Agreement Dissection April 28, 2011 Mark Cendrowski After Sheldon barges into the bathroom and finds Leonard and Priya in the shower together in violation of the Roommate Agreement, Priya as a lawyer reviews it and finds loop holes in the document.
85 422 The Wildebeest Implementation May 5, 2011 Mark Cendrowski Penny and Amy use Bernadette, codename Wildebeest, to spy on Leonard and Priya at a dinner party she and Howard are attending.
86 423 The Engagement Reaction May 12, 2011 Howard Murray Bernadette has lunch with Howard's mother who ends up in the emergency room immediately after learning of their engagement. Penny and Priya start bonding over Leonard's quirks.
87 424 The Roommate Transmogrification May 19, 2011 Mark Cendrowski Leonard and Priya decide to live together in Raj's Apartment, while Raj can become Sheldon's roommate. Sheldon is very happy after Raj prepares dinner for them. Penny and Raj get drunk together and crawl into bed together. 

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