Description This backwater Valve 4.0″ is for use with 4″ Pipe. This valve is a backwater prevention device used to prevent outbound water through a dwelling’s drain pipes from re-entering — “back flowing”—into a home. The valve contains a flap that allows water to exit the home, but closes to prevent the back flow into the home. This backwater Valve is for use with 4.0″ Pipe