Four-way valve

The four-way valve, or four-way cock, is a fluid control valve whose body has four ports.
One commonly used form—the rotary four-way valve—has equally spaced ports around the valve chamber and a plug with two passages that connect pairs of adjacent ports. The plug may be cylindrical, tapered, or a ball. It has two flow positions, and usually a central position where all ports are closed.
An application of this design is 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.
Another common use case is to reverse the flow of fluid through some portion of a system. An example is a reversible heat pump, where a four-way valve reconnects the two heat exchangers to be either evaporator or condenser. It was used to control the flow of steam in the valve gear of early double-acting steam engines, such as those designed by Richard Trevithick. This use of the valve is possibly attributable to Denis Papin.
Because the two passages in the plug are somewhat L-shaped and do not interconnect, the four-way valve is sometimes called an × port.