A 4x4 actuator is a mechanism that engages the front differential to put the vehicle into four-wheel drive. A 4X4 actuator is an electric component as opposed to a manual lever used to engage the front axle of the vehicle.
The biggest advantage of using the electric 4x4 actuator is the ability to switch from two-wheel to four-wheel drive without having to stop and shut off the vehicle.
The disadvantage to having an electric 4x4 actuator rather than a manual 4x4 system is that sometimes it can take several minutes for the device to warm up enough to engage the front differential. This can cause the vehicle to become stuck in areas where four-wheel drive is suddenly necessary. There is a 4x4 actuator available that is fully electric and does not require a warm-up period. This option engages the 4x4 immediately and is very useful for those who often drive off-road.
Another type of actuator is operated by a vacuum. This is the least-popular device, and owners often choose to replace it with a manual 4x4 system. The major issue that arises with a vacuum-operated actuator is that it can develop problems creating a seal and locking the front axle into place.