Calling all plumbers... I am installing a 2nd bathroom in my basement. I currently have one bathroom on the main floor of a ranch home. The whole dwv system is 4 inch cast iron. I have busted up the basement floor to access the main pipe heading to the sewer. I have purchased a sanitary y to tie into the 4 inch pipe. My new stool will be about 2 feet from the junction point under the floor. I bought a 4 inch y with a 3 inch inlet on the added arm. All the new plumbing, for a 3/4 bath, will be pvc. My question is should I have stayed with 4 inch or will I be ok with 3" for the additional bath? The only reason I went with 3 was that my local store seemed to have a better selection of 3 inch fittings.