Duplex with Two Bonus Apartments. How do I make the most of it?

[Rick DiAgostino]

I am looking into a two family home that has two "bonus apartments", essentially making it a four family home on a two family lot. Each unit has its own separate entrance but the home has a two family CO and is only zoned for two family. I have been seeing a lot of similar postings stating that there is room for an extra unit or a "bonus unit". How would someone take advantage of a theoretical "bonus unit"? How would the extra unit get mail? Could it be sub metered? Any and all advice is appreciated.