Owning a 4-family vs. 3- & 2-family: extra steps?

Looking to close and move into a house that will continue as a 4-family.  I know NYC considers 4-fams to jump into the multi-unit class, and that things may have to be different than in a 3-family.  but that’s about all I know so far. Does anyone know of a clear, plain-english resource (cheat sheet doc, blog post, web page etc) that summarizes the various extra hurdles I will need to go through, burdens I should expect from city etc. that might not be obvious to me from my limited experiences living in 2- and 3-  family units?