A valve body has a piston that slides within the body through four successive positions. The piston has a head, and an upper and lower skirt, with a port in the upper skirt. In the first position, a bias force urges the piston to the first of four positions, in which the piston port is closed. In a second position, the piston port aligns with a low pressure port in the valve body, when a low pressure supply of water is connected. In a third position, greater pressure again closes the piston port. In a fourth position, at a still greater pressure, the piston port aligns with a high pressure port in the valve body. The bias force or a location of the piston port can be varied for valves along a supply line, whereby varying supply pressure opens different valves, thereby enabling addressing of valves according to supply pressure.