I need to add a sink drain to a 3" soil stack. I don't have room to add a wye above the 3" to 4" cast iron reducer (B). Can I eliminate the cast iron reducer (B) and and use a shorter fitting to take the 3" ABS directly into the 4" cast cleanout (C)? What kind of problems might I encounter removing the reducer (B)? Here is the link to a picture of the stack. https://www.dropbox.com/s/5m4aqiuuh6nst54/Soil%20Stack%201-wc.jpg