Brett_Murphy has the best definition/description. 4X4 can be either part-time or full-time. I was told during my 4X4 days that the difference between the terminology is intended use. 4X4 is intended for off-road and what is referred to as AWD is intended for on-road. AWD has dif or some sort of slip for pavement use. 4X4 will lock/engage more solidly to work in slippery conditions. Even if full-time with a dif in the middle. Too much pavement, especially dry pavement will tear up a 4X4 engaged. Drive shafts and U-joints are usually first to go but bushings, gears and other stuff will wear out earlier than if used as designed or a 2WD variant.
