There are several other factors that come into play which will determine if it is legal for you to make said alterations. In order to find out what you can or cannot do you need to speak to an architect. If I can be of further assistance, please let me know. Good luck!
Mitchell S. Feldman
Associate Broker/ Director of Sales
Madison Estates & Properties, Inc.
Office: (718) 645-1665/ Cell: (917) 805-0783
Dept of City Planning
Zoning > Zoning Districts - Residence Districts > R4