It sounds like you are looking for a legal 3 family with a "bonus" unit. That is NOT a legal 4 family and the mortgage company is going to have issue with counting the income from the "bonus" unit since it is not a legal apartment.
You may want to just look at legal 4 family units where all of the rental income can be used by your mortgage company.
Please let me know if you have any additional questions. I'll be happy to assist.
Walter J. Burns
1 Newark St.
Hoboken, NJ 07030
201-653-8488 Ext: 230