The seller answered my question by explaining that "There is a 4th wheel designed into the car. It is immediately behind the large rear wheel, but not in place for the photos. I designed the car that way so that I could run more than one class. SCTA rules not only require four wheels for a car, they also require that two of the wheels be driven. There is provision for driving the 4th wheel as well. Hope that helps."
He also provided some contact information.
Anyway, that sounds like a pretty good answer