I am in the process of renovating my basement and will be building a new bathroom as well. The contractor already installed the drains for a shower, a toilet and a sink and a 4" backflow valve (the Bow model with a trap door) because there was none previously.Now, since the old plumbing is exposed, I want to gain a bit of knowledge before I continue the renovations and call back the contractor. There is one pipe in particular that is a mystery for me. The old sink was about 3 1/2 - 4 feet from the stack. The pipe from the sink to the stack has a small back flow valve and another vertical pipe that goes up through the floor. This vertical pipe does not seem to be a drain from an installation on the floors above. Is it the vent? I thought at 4 feet the vent was not necessary. Has anyone seen a similar setup? If it's a vent and can be used for the new bathroom,I don't want the contractor to say he has to pass a new vent ($$$$$) or not connect to a vent whatsoever. See the attached picture. The dollar sign on the picture points to the main stack... it's an editing mistake, not an allegory (like "money down the drain").