This valve is quite similar to 4/2 way spool valve. Schematic diagram is shown in Figure.
In this design disc is used instead of a spool. This suspended disc can be moved by pilot pressure or by solenoid or by mechanical means. In this design, main disc connects port 1 to either port 4 or 2. The secondary seat discs seal the exhaust port 3 whichever is not functional. These values are generally provided with manual override to manually move the cylinder.