4WD distributes power to all four wheels, but they will not necessarily be powered at the same time. Different 4WD systems include:
1) Full-time 4WD (or permanent 4WD): Power is constantly provided to all four wheels, usually with a power shift between the front and rear axles whenever necessary. Under Full-time 4WD, drivers can enjoy maximum traction regardless of driving conditions (dry or slippery). Additional action is not required for its activation.
2) Part-time 4WD: With part-time 4WD, drivers have to shift between the selectable 2WD and 4WD provided by the vehicle, usually with a lever or a switch - and part-time 4WD allows a driver to do this while driving. The lock between front and rear allow drivers better traction on slippery surfaces. While it is the best option for nearly all off-road conditions, driving with part-time 4WD on dry and/or smooth roads is not recommended.
3) Automatic 4WD: While the default is 2WD (front or rear), a full-time system will judge when 4WD or AWD is needed and automatically direct power to all four wheels - varying the distribution between the front and rear axles if needed. A slipping wheel is the most common activator of the shift, but a more sophisticated system will make the shift ahead of that slip.