In my old apartment building my floor (10 apartments) had the same +4. Now in my new condo, my whole side of the street had the same +4.

As for the main 5 digit Zip, my city has 10 of them, but the next town only has one for the whole town, which is about half the size, but less populated. I have also heard about single buildings or companies getting their own 5 digit zip code if they receive allot of mail.