The primary difference between four-wheel-drive vehicles and all-wheel-drive vehicles is that all-wheel drive systems are always active, while most four-wheel-drive systems require manual activation. However, not all all-wheel-drive systems provide power to all four wheels at all times.
Four-wheel drive is the oldest method for powering all four wheels of a car or truck. Four-wheel-drive systems direct power from the engine to a transfer case that then sends power to all four wheels when the system is active. However, the design of four-wheel-drive mechanisms splits power evenly to all four wheels, which can make it more difficult to maneuver the vehicle. For this reason, manufacturers design most four-wheel-drive systems for manual activation when maximum traction is required. When the system is not active, a four-wheel-drive vehicle operates like a normal two-wheel-drive car or truck.
In contrast, all-wheel-drive systems are always active but use a combination of electronic and mechanical systems to distribute engine power to the wheels that have the most traction at a given time. This results in more efficient use of engine power and better handling under most conditions, though all-wheel-drive systems are still less efficient under very low-traction conditions as compared to four-wheel-drive systems. While some all-wheel-drive systems direct some power to all four wheels at all times, other designs primarily power the front or rear wheels and only direct power to the other wheels when necessary.