This valve is a pilot to open check valve.
It has a sealed pilot, a steel seat, and is vented. It allows free flow from the valve (port 2) to the load (port 1) and blocks flow in the opposite direction.
Pressure at the pilot (port 3) will open the valve from port 1 to port 2.
Pilot pressure needed to open the valve is directly proportional to the load pressure
at port 1.
The valve is insensitive to pressure at port 2 because the spring chamber is referenced to the vent (port 4).