Sheldon says the address of their apartment is 2311 North Los Robles Avenue in Pasadena, California. However, where Leonard lies to Sheldon about going across the street to use the gas station bathroom, they note that their building is near Los Robles Ave. and Colorado.
On the day Bernadette and Howard were to be married, they chose a quickie wedding on the rooftop of Sheldon and Leonard's apartment. It's in this episode where they say their apartment building's address is 215 South Madison Avenue.
When you say 4A it sounds like you're saying foray. Foray could mean the actual definition, which is raiding an enemy's territory or it could be a special shoutout to June Foray, the voice of Rocky the Flying Squirrel.
Sheldon explains he got rid of them because of how carcinogenic they were.
Sheldon would never dream of taking his clothes off in public (especially a communal laundry room), there's a sign hanging in the laundry room that says "Please keep your clothes on while doing laundry."
If you look closely when Sheldon and Leonard are going through tough times, the roommate flag magnet is typically turned upside down on the fridge.
To honor Carol, there is a small picture of her hanging on Leonard and Sheldon's fridge.
It's quite possible that there were apartments in the main lobby — we've just never seen them.
We see the two staircases, the two apartments, and the elevator door.
We'll never know.