Season 4 (2010)
This season the Big Bang gang’s romantic universe expands. On the rebound from Penny, Leonard falls into the arms of Raj’s sister Priya. Sheldon gets a girlfriend, or rather a friend who is a girl: Amy, a dour neurobiologist who declares herself besties with Penny. Howard and Bernadette heat up. And so do Raj and Bernadette (at least in Raj’s Bollywood daydream).
Ep.1 The Robotic Manipulation
Air Date: 2010-09-23
Penny tags along on Sheldon's first ever date; Wolowitz finds a new use for a robotic arm.
Ep.2 The Cruciferous Vegetable Amplification
Air Date: 2010-09-30
Realizing he will not live long enough to download his consciousness into a robot body, Sheldon tries to extend his life span.
Ep.3 The Zazzy Substitution
Air Date: 2010-10-07
The guys are concerned as Sheldon searches for an alternative to human companionship.
Ep.4 The Hot Troll Deviation
Air Date: 2010-10-14
An embarrassing secret of Wolowitz's comes to light, while Sheldon and Raj have a small war at work.
Ep.5 The Desperation Emanation
Air Date: 2010-10-21
Leonard realizes he’s the only one without a girlfriend, while Sheldon wants to get rid of his.
Ep.6 The Irish Pub Formulation
Air Date: 2010-10-28
Leonard keeps an affair a secret from Sheldon, Howard and Raj.
Ep.7 The Apology Insufficiency
Air Date: 2010-11-04
Sheldon's answers during an FBI interview put Wolowitz's security clearance in jeopardy.
Ep.8 The 21-second Excitation
Air Date: 2010-11-11
While the guys camp in line for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party.
Ep.9 The Boyfriend Complexity
Air Date: 2010-11-18
Penny asks Leonard to lie to her father, while Howard, Raj and Bernadette pull an all-nighter at a telescope.
Ep.10 The Alien Parasite Hypothesis
Air Date: 2010-12-09
Amy finds she has sexual feelings for Penny's ex, while Koothrappali and Wolowitz try to prove who would be the better superhero.
Ep.11 The Justice League Recombination
Air Date: 2010-12-16
The guys find a "super" use for Penny's new boyfriend, Zack, when they enter a costume contest as the Justice League.
Ep.12 The Bus Pants Utilization
Air Date: 2011-01-06
Leonard's idea for a smartphone app derails his friendship with Sheldon.
Ep.13 The Love Car Displacement
Air Date: 2011-01-20
Bernadette runs into her ex-boyfriend while attending a science event with Wolowitz.
Ep.14 The Thespian Catalyst
Air Date: 2011-02-03
Hoping it will make him a better teacher, Sheldon has Penny give him acting lessons. Raj has fantasies about a friend's girlfriend.
Ep.15 The Benefactor Factor
Air Date: 2011-02-10
A wealthy donor to the university makes Leonard consider how far he's willing to go for the sake of science.
Ep.16 The Cohabitation Formulation
Air Date: 2011-02-17
As Wolowitz contemplates taking things with Bernadette to the next level, Leonard rekindles his relationship with Raj's sister.
Ep.17 The Toast Derivation
Air Date: 2011-02-24
Sheldon becomes concerned when everybody begins to hang out at Raj's apartment and discovers Leonard is the center of the group.
Ep.18 The Prestidigitation Approximation
Air Date: 2011-03-10
Sheldon is baffled by Wolowitz's magic trick. Meanwhile, Leonard realizes dating Priya may make it impossible to continue being friends with Penny.
Ep.19 The Zarnecki Incursion
Air Date: 2011-03-31
Sheldon's online-game account is hacked by Todd Zarnecki, who then proceeds to steal all his items prompting the guys to go on a quest to retrieve them. Penny, Bernadette and Amy discuss how Penny groomed Leonard to be willing to try various things and more adventurous boyfriend... for some other woman — Priya.
Ep.20 The Herb Garden Germination
Air Date: 2011-04-07
Sheldon and Amy decide to start several rumors about the others as a test. Meanwhile, Howard takes his relationship with Bernadette to even higher level.
Ep.21 The Agreement Dissection
Air Date: 2011-04-28
When Leonard spends too much time in the shower with Priya, Sheldon charges Leonard with two violations of the roommate agreement. However, before he can get too far, Priya uses her skills as a lawyer to nullify the charges due to the agreement's vague wording, much to Sheldon's dismay. When the guys take advantage of Priya's skills in order to eat Greek food on pizza night, Sheldon goes to Penny, who invites him to a girls' night out with Amy and Bernadette, which Sheldon agrees to after Penny mentions that they may be talking trash about Priya. After the girls drink some cocktails, they decide to take Sheldon dancing, which Sheldon reluctantly goes along with. After dancing, Sheldon takes an intoxicated Amy back to her apartment and as well as talking about Priya, meets a monkey who is addicted to smoking cigarettes. Amy tells Sheldon not to give in to Priya's law skills, before kissing him and going to her bathroom to vomit.
Ep.22 The Wildebeest Implementation
Air Date: 2011-05-05
Raj attempts to cure his social anxiety disorder while Penny uses Bernadette to spy on Leonard and his girlfriend.
Ep.23 The Engagement Reaction
Air Date: 2011-05-12
When Howard finally breaks the news to his mom that he's engaged to Bernadette, it lands her in the hospital.
Ep.24 The Roommate Transmogrification
Air Date: 2011-05-19
When Bernadette receives her PhD, Wolowitz feels emasculated. Raj and Leonard swap places and Penny does something she's regrets.