I am looking for some expertise about plumbing so I can have an idea when I talk with the plumber about dwv layout From looking at dfu loads a vertical stack of 3 inches will support 4 bathroom groups but some people say it needs to be 4 inch vertical if 4 or more wc are involved. I was planning a horizontal line of 4" leaving the house but would prefer 3" vertical if okay. So main question:

Can I use 3" vertical stack if within vertical dfu limits with 4 wc involved? I want to leave room for expansion and don't want to replace major plumbing when I add the 4th bathroom later on. A link to code posting would be awesome too. Thanks