The Fast Fours is a simple variation on a standard set-of-fours. The difference is that the far end of the rope is not terminated on the becket of a pulley. Instead it is run through another progress capture pulley. This means that an operator can either pull on both ends simultaneously to yield a fast take up 2:1 or a single strand for a 4:1.
There is another unique advantage of this system for fallen worker and confined space recovery. If capable, the fallen worker can pull on one end of the system while the rescuer pulls on the other which speeds the recovery, halves the individual work, and involves the victim in the task.