You have 4WD if you have 4 driven wheels (and unless you count the spare, that's all my wheels). Then, you can have a multitude of differential configurations on the 3 differentials (open, lsd, locker, diff-less viscous clutch, diff-less locker).
A Jeep WK uses a locking differential center, and electric engaging LSD's front and rear. By that article, the car is AWD on road, and sorta 4WD when off. The Jeep WJ is either controlled by a viscous clutch (Quadra-Drive), or has manually selectable 2WD, 4WD open ("AWD") and 4WD locked ("4WD") modes. A 98 Ford Explorer pulses an electric clutch to go between 2WD and 4WD locked. These cars all behave very differently both on and off road, and they don't fit well in the authors categories.
If you're buying or owning a car that drives all wheels, the only useful information is the diffs. Labels like "4x4", "4WD", "AWD", "Quadra-Trac", and "Quattro" mean nothing.
Most new 4WD's use open diffs with electrically controlled lockers or LSD's, as that gives the best combined on/offroad experience. Permanent LSD's/viscous clutches (Jeep WJ, for example) is a bit more responsive in surprise low-traction conditions, but it eats your tires when you make tight u-turns, as the turn will progressively engage a complete diff lock. Center lock without diff works like a regular 2WD until you lock, but remember that if you have ANY type of locker on ANY axle (not LSD), then engaging in high traction conditions might snap your axels in a very loud and explosive manner, even if you think you're driving straight.
You have 4WD if you have 4 driven wheels (and unless you count the spare, that's all my wheels). Then, you can have a multitude of differential configurations on the 3 differentials (open, lsd, locker, diff-less viscous clutch, diff-less locker).
A Jeep WK uses a locking differential center, and electric engaging LSD's front and rear. By that article, the car is AWD on road, and sorta 4WD when off. The Jeep WJ is either controlled by a viscous clutch (Quadra-Drive), or has manually selectable 2WD, 4WD open ("AWD") and 4WD locked ("4WD") modes. A 98 Ford Explorer pulses an electric clutch to go between 2WD and 4WD locked. These cars all behave very differently both on and off road, and they don't fit well in the authors categories.
If you're buying or owning a car that drives all wheels, the only useful information is the diffs. Labels like "4x4", "4WD", "AWD", "Quadra-Trac", and "Quattro" mean nothing.
Most new 4WD's use open diffs with electrically controlled lockers or LSD's, as that gives the best combined on/offroad experience. Permanent LSD's/viscous clutches (Jeep WJ, for example) is a bit more responsive in surprise low-traction conditions, but it eats your tires when you make tight u-turns, as the turn will progressively engage a complete diff lock. Center lock without diff works like a regular 2WD until you lock, but remember that if you have ANY type of locker on ANY axle (not LSD), then engaging in high traction conditions might snap your axels in a very loud and explosive manner, even if you think you're driving straight.