Converting a Legal 2-Unit to a 4-Unit Property in Brooklyn, NY

[Cameron Best]

I'm currently looking at buying a legal 2-unit property in Brooklyn, NY. The property is two-stories plus a basement and is quite spacious. I was originally looking at this as a gut renovation of a two units, but recently started thinking about converting the two above ground floors into 4 legal units and living in the basement as a house hack. Being that this is New York, I know the permitting can be a challenge - I have looked up the zoning and it is R6, so I don't anticipate there being any zoning issues. 

I'm mostly trying to assess what additional steps I would need to take to convert if to a 4-unit property, if I was already planning to gut it and renovate it to begin with as a 2-unit. Is it two additional permits and C/Os? Or will there be lots of new requirements? 

I would be very interested in hearing from anyone in NYC who has done this.