Converting triplex into quadplex - Philadelphia, PA

[Mark Albini]

Hey guys! First time posting on the BP forum and looking forward to your responses. 

I found a triplex (zoned RSA-5) and after running the numbers it runs with the three units but it kills with four. I was thinking of converting the basement into a separate unit but wasn't sure if that'd be possible with the current zoning. Hope someone can provide some insight!