Apartment 5A is Alicia's apartment, directly above Sheldon's and Leonard's apartment (4A). She moved in during "The Dead Hooker Juxtaposition", and the apartment has appeared again in "The Tenant Disassociation".
Sheldon was terrified at the prospect of new upstairs tenants whom he had never met, saying he preferred the old ones because they were always quiet. Alicia was making quite a bit of noise upstairs while supposedly having sex with a producer from CSI. They don't mention Apartment 5A in the previous episodes, or again until season 11. Then the configuration is different than it was in "The Tenant Disassociation".
The apartment is almost identical in shape and configuration to the apartment that Sheldon and Leonard live in.