Apartment 4B is Penny's former and now Sheldon's and Amy's apartment and the secondary setting for The Big Bang Theory. She lived alone in the apartment until her husband Leonard moved in, but they still spend most of their nights in Apartment 4A due to Sheldon having had a hard time with Leonard moving out. It is located on the fourth floor of the Building at 2311 North Los Robles Avenue, Pasadena, California. When viewed from the satellite photographs in "The Countdown Reflection" episode it appears to be located at 215 South Madison Avenue. It is most well known as being located directly across the hallway from Apartment 4A, which is the apartment that Sheldon lives in and in which Leonard also used to live in with Sheldon as roommates though the married couple spent many nights there. Sheldon and Amy used it for a five-week experiment in living together and then formally moved in together.
4B is a smaller apartment than its counterpart across the hall, having only a single bedroom instead of two; however, like its counterpart across the hall, it has an open floor plan where the living room and the kitchen are part of the same room. Also like 4A, the apartment is entered into the living room through the hallway. The right wall has a single door that leads into Penny's bedroom. Continuing along the wall is a large window with her kitchen table by it. The bathroom in the apartment is located through her bedroom, and is the only way to access the bathroom.
The wall on the left immediately after you enter the apartment is not visible, as it is the view into the apartment when episodes are being filmed. The wall houses Penny's TV and entertainment center and her portrait of her and Amy Farrah Fowler, and ends at the kitchen area.
In Penny's kitchen, there is an island with chairs as well as a small window over the sink.
Apartment 4B is first seen in the pilot, upon meeting Penny, who is the newest tenant in the building. We also find out that the former occupant of 4B was "a 200 pound transvestite with a skin condition". This former occupant's name was revealed as Louie/Louise in the following episode called The Big Bran Hypothesis. The Big Bran Hypothesis also gives us the first view inside the apartment, in which Sheldon and Leonard help move in a media set while Penny is at work. Sheldon describes Louie/Louise as being cleaner than Penny, having helped him run cable for a web-cam while he lived there.
Trivia Note: When Leonard first sees Penny through the doorway in the Pilot, the kitchen should be visible, but is not. Only the coffee table that is still in front of her couch can be seen in her full apartment set.
Penny's Apartment is featured in several episodes:
- Pilot - Only seen briefly.
- The Big Bran Hypothesis - Sheldon cleans her apartment which freaks Penny out.
- The Grasshopper Experiment - Penny mixes drinks where Raj is able to talk to her for the first time by being drunk.
- The Peanut Reaction - Preparing for Leonard's birthday party.
- The Tangerine Factor - Penny's breakup with Mike/Leonard's help/also talking to Sheldon.
- The Barbarian Sublimation - Penny plays Age of Conan and becomes hooked on it.
- The White Asparagus Triangulation - Sheldon questions Penny.
- The Bath Item Gift Hypothesis - Leonard is angry about David Underhill.
- The Killer Robot Instability - Leonard begs Penny to talk to and forgive Howard, who is a hermit, and wouldn't leave his house.
- The Financial Permeability - Penny's financial crisis/electricity has been cut off as a result.
- The Maternal Capacitance - Penny and Leonard try to avoid Leonard's mother.
- The Work Song Nanocluster - Penny starts a business making Penny Blossoms.
- The Hofstadter Isotope - Sheldon & Stuart argue over Batman's replacement.
- The Vegas Renormalization - Sheldon locks himself out of his apartment.