Pictures if possible and it's wet vented. You can drain a tub and sink into a stack and vent the toilet, sink and tub through that entire stack. If its not going to outside air that's very surprising. If its 4" you should technically increase one size before you penetrate the roof, so a 4x6 coupling. At least in my area that's what we do.
If you don't learn something new everyday at work, it's not even worth going.