Assuming you are referring to a Four Wheel Drive (4WD) system in an automobile:
A 4WD system allows all four of a vehicle's wheels to receive power from its engine simultaneously. Most 4WD systems engage and disengage the front and rear axles automatically, depending on traction conditions. Some systems have an additional low range gear for increased torque at low speeds, which is engaged manually by the driver.
In order for a 4WD system to work, there must be some way to engage and disengage the front and rear axles. This is typically done with an actuator, which is a device that converts energy into motion. The most common type of actuator used in 4WD systems is hydraulic, but electric and pneumatic actuators are also sometimes used.
Hydraulic actuators use fluid pressure to generate force. A pump generates this pressure, which is then sent to one or more cylinders that move the axle shafts. Electric actuators use electromagnets to generate force. These magnets are activated by electric current, which creates a magnetic field that interacts with the magnets in the axle shafts to rotate them. Pneumatic actuators use compressed air to generate force. A compressor pumps air into a cylinder, which then pushes on a piston that rotates the axle shafts.