What do you call a fast four-beat gait?

The term that refers to a fast four-beat gait is "Rack." In equestrian terminology, the Rack is a specific gait characterized by its smoothness and the distinct timing of the footfalls, where each hoof strikes the ground separately in a quick succession. This gait is often seen in certain breeds, like the Tennessee Walking Horse, which is known for its high stepping, flashy movement that has both speed and elegance.

In contrast, a Canter is a three-beat gait known for its moderate speed, while a Gallop is a faster, four-beat gait but typically combines a different rhythm where the horse has a more pronounced leap, making it less smooth than the Rack. The Walk, being a slower and four-beat gait, does not fit the description of being 'fast,' which further differentiates the Rack. Thus, "Rack" is specifically recognized for both its speed and its four-beat cadence, making it the correct answer.