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).

Season 4 Episode 10 - The Alien Parasite Hypothesis

Amy finds she has sexual feelings for Penny's ex, while Koothrappali and Wolowitz try to prove who would be the better superhero. Air Date : 9th-Dec-2010

Season 4 Episode 1 - The Robotic Manipulation

Penny tags along on Sheldon's first ever date; Wolowitz finds a new use for a robotic arm. Air Date : 23rd-Sep-2010  Read More

Season 4 Episode 2 - The Cruciferous Vegetable Amplification

Realizing he will not live long enough to download his consciousness into a robot body, Sheldon tries to extend his life span. Air Date : 30th-Sep-2010  Read More

Season 4 Episode 3 - The Zazzy Substitution

The guys are concerned as Sheldon searches for an alternative to human companionship. Air Date : 7th-Oct-2010  Read More

Season 4 Episode 4 - The Hot Troll Deviation

An embarrassing secret of Wolowitz's comes to light, while Sheldon and Raj have a small war at work. Air Date : 14th-Oct-2010  Read More

Season 4 Episode 5 - The Desperation Emanation

Leonard realizes he’s the only one without a girlfriend, while Sheldon wants to get rid of his. Air Date : 21st-Oct-2010  Read More

Season 4 Episode 6 - The Irish Pub Formulation

Leonard keeps an affair a secret from Sheldon, Howard and Raj. Air Date : 28th-Oct-2010  Read More

Season 4 Episode 7 - The Apology Insufficiency

Sheldon's answers during an FBI interview put Wolowitz's security clearance in jeopardy. Air Date : 4th-Nov-2010  Read More

Season 4 Episode 8 - The 21-Second Excitation

While the guys camp in line for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party. Air Date : 11th-Nov-2010  Read More

Season 4 Episode 9 - The Boyfriend Complexity

Penny asks Leonard to lie to her father, while Howard, Raj and Bernadette pull an all-nighter at a telescope. Air Date : 18th-Nov-2010  Read More

Season 4 Episode 10 - The Alien Parasite Hypothesis

Amy finds she has sexual feelings for Penny's ex, while Koothrappali and Wolowitz try to prove who would be the better superhero. Air Date : 9th-Dec-2010  Read More

Season 4 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. Air Date : 16th-Dec-2010  Read More

Season 4 Episode 12 - The Bus Pants Utilization

Leonard's idea for a smartphone app derails his friendship with Sheldon. Air Date : 6th-Jan-2011  Read More

Season 4 Episode 13 - The Love Car Displacement

Bernadette runs into her ex-boyfriend while attending a science event with Wolowitz. Air Date : 20th-Jan-2011  Read More

Season 4 Episode 14 - The Thespian Catalyst

Hoping it will make him a better teacher, Sheldon has Penny give him acting lessons. Raj has fantasies about a friend's girlfriend. Air Date : 3rd-Feb-2011  Read More

Season 4 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. Air Date : 10th-Feb-2011  Read More

Season 4 Episode 16 - The Cohabitation Formulation

As Wolowitz contemplates taking things with Bernadette to the next level, Leonard rekindles his relationship with Raj's sister. Air Date : 17th-Feb-2011  Read More

Season 4 Episode 17 - The Toast Derivation

Sheldon becomes concerned when everybody begins to hang out at Raj's apartment and discovers Leonard is the center of the group. Air Date : 24th-Feb-2011  Read More

Season 4 Episode 18 - The Prestidigitation Approximation

Sheldon is baffled by Wolowitz's magic trick. Meanwhile, Leonard realizes dating Priya may make it impossible to continue being friends with Penny. Air Date : 10th-Mar-2011  Read More

Season 4 Episode 19 - The Zarnecki Incursion

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. Air Date : 31st-Mar-2011  Read More

Season 4 Episode 20 - The Herb Garden Germination

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. Air Date : 7th-Apr-2011  Read More

Season 4 Episode 21 - The Agreement Dissection

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. Air Date : 28th-Apr-2011  Read More

Season 4 Episode 22 - The Wildebeest Implementation

Raj attempts to cure his social anxiety disorder while Penny uses Bernadette to spy on Leonard and his girlfriend. Air Date : 5th-May-2011  Read More

Season 4 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. Air Date : 12th-May-2011  Read More

Season 4 Episode 24 - The Roommate Transmogrification

When Bernadette receives her PhD, Wolowitz feels emasculated. Raj and Leonard swap places and Penny does something she's regrets. Air Date : 19th-May-2011  Read More