What number is Sheldon’s apartment?

Apartment 4A is the primary permanent set for The Big Bang Theory, and is the apartment where Sheldon Cooper lived until "The Cohabitation Experimentation" when he moved into Apartment 4B with his girlfriend, and eventual wife, Amy. At first it was only for five weeks due to Amy's apartment being flooded but Sheldon and Amy chose to live there permanently. Leonard Hofstadter has lived in this apartment since 2003 and currently lives there with his wife Penny. It is situated in an apartment complex at 2311 North Los Robles Avenue. By season 8, after Penny and Leonard's engagement, she was seen living there most of the time though not officially moved in. When viewed from the satellite photographs in "The Countdown Reflection" episode, it appears to be located at 215 South Madison Avenue. Due to the primary characters in the series, except Penny, being of the "nerd" social classification, the apartment has been dubbed as "Nerdvana" by Howard in season 1 episode 14 "The Nerdvana Annihilation". Apartment 4A is located on the fourth floor of the building across the hall from Apartment 4B, where Penny lived and where Sheldon and Amy live now. Apartment 4A is known to be a corner apartment, being that it has windows facing outside on two sides. It may, in fact, be one of only two apartments on the floor, and may therefore have outside walls on all sides (other than the wall shared with the fourth floor hall and stairwell. The mailboxes in the lobby, however, indicate there are four units per floor, with only four floors in the building. The presence of unit 5A in season 2 however makes the mailboxes make absolutely no sense.