Actually,
US States do not have the legal ability to change a US vehicle's NHTSA certified VIN classification from the manufacturer. I checked into all the legal's on this when I did my quad kit last year.
Only the NHTSA can classify a vehicles' VIN.
Even if you wanted too... Good luck getting a Slingshot to pass all the NHTSA automobile safety requirements.. bumpers, airbags, windshield wipers, heater, crash ratings, etc...etc....
A 4 wheeled Slingshot is no different than a motorcycle with extra wheels for safety. They are everywhere.
A VIN classified Motorcycle (certificated by the NHTSA, as so, for the manufactor) could be modified by a owner to have dozens of wheels and even a 1980's style hut tub attached to the rear trunk area.... and its still a MOTORCYCLE.
Only saying. because I did all my homework on this.
Webby