the four inch pipe is the main drain line from the upstairs of the house going behind the dishwasher. the dishwasher already has an anti syphon kit installed in sink [it's clean]. customer had plumber come out to house and he said the air vent pipe going through the roof had a dead squirrel in it, causing some drains throughout the house to syphon. after he cleared the air vent pipe on the roof everything works fine. thanx for the tips guys.