A FOUR-WHEEL alignment is a specific alignment procedure where both front and rear wheels are adjusted in relation to the thrust line.   Essentially, the THRUST ANGLE is adjusted first (usually to zero degrees) by adjusting TOE at the rear wheels, then the front wheel toe angles are set to compensate for any variation of the thrust line from the center line of the vehicle.  This results in the rear wheels tracking correctly behind the front wheels with no DOG-TRACKING, as well as the steering wheel in the correct position.

Other alignment types are THRUSTLINE Alignment and CENTERLINE Alignment.
More detailed informations about THRUST ANGLE and other wheel alignment angles will be available soon.