The four-way valve or four-way cock is a fluid control valve whose body has four ports equally spaced round the valve chamber and the plug has two passages to connect adjacent ports. The plug may be cylindrical or tapered, or a ball.
It has two flow positions as shown, and usually a central position where all ports are closed.
It can be used to isolate and to simultaneously bypass a sampling cylinder installed on a pressurized water line.
It is useful to take a fluid sample without affecting the pressure of a hydraulic system and to avoid degassing (no leak, no gas loss or air entry, no external contamination).