The "main vent" has nothing draining down from a story above? (If you answered this already, I've lost track). If so I would be sorely tempted to use a plain double wye, on the basis that 4" downstream is good enough for the vent. And that the arrangement would be allowed under the IPC (not applicable to you).It just seems silly require a vented fixture when butted to another vented fixture that has excess capacity. The proposed new vent will have less than a 4 inches (vs the existing 9.6 inches) from the main vent, but it is what it is.