You might think that four-wheel-drive (4WD) and all-wheel-drive (AWD) would be the same thing but, as George Gershwin once noted, it ain't necessarily so.
Four-wheel-drive has come to mean a vehicle with low-range gearing. When low range is chosen, drive goes to the wheels via another set of gears that multiplies by double or more the "pulling power" of each normal ratio.
In first-gear low range, the vehicle crawls along at a walking pace and has power to get a loaded trailer moving or to drag heavy objects.
Low-range gearing also offers the driver better control in challenging conditions, such as off-road tracks and trails. But because the gearing is so much lower, the vehicle's top speed will be cut significantly.
Many want the advantages of having all wheels driven when launching a boat, driving to ski fields or on wet, slippery suburban roads.