Lot Area. The lot area and frontage regulation applicable to single-family and two-family dwellings shall be the same as those in the R-1 or R-2 District, as the case may be. For three-family residences, there shall be a lot area of not less than seven thousand five hundred (7,500) square feet, and all lots for such use shall have a frontage upon a public street of not less than seventy-five (75) feet. The minimum lot area for four-family residences shall be ten thousand (10,000) square feet; the minimum frontage requirement for such use shall be one hundred (100) feet. Nothing herein shall be construed as to permit the division of an existing lot for the construction of a building unless all requirements with respect to area and yard are complied with. Further, no structure in existence as of the effective date of Part 3, Zoning, may be converted to any use permitted in this district unless the lot complies with the area requirements provided for herein and does not increase any existing legal nonconformance as to be front, side or rear yard or otherwise violate front, side or rear yard regulations contained herein.