Currently we have a 4"cast iron main stack/drain pipe. We are putting in a new bathroom, and going to be taking out the old main vent. We are wanting to just use a 2" main vent in the wall since the current walls are only 2x4. All that would be going off this vent would be the toilet, sink, and tub then going to the sewer line. We will be tapping back into this 4" pipe in the attic. Eventually we will be redoing the kitchen and it will have it's own 2" vent going to the sewer and attic. Will this work, or cause problems. Thanks much.