No, no, no, no, NO!!!!!!! You're tempting the heavy hand of the bureaucratic safety ninnies.
Spyder riders enjoy a grey area of automotive safety regulation because the Spyder has three wheels. That's generally accepted as an extension of sidecar rigs and Harley-like trike conversions.
But to the automotive safety bureaucrat, four wheels = car, and all the safety considerations must come into play: rollover safety, airbags, crumple zones, etc. That's why the BMW GG Quad mentioned above initially met resistance over here. See also the
BMW Just42.
Vehicles like the Polaris Slingshot are blurring the distinction -- that's why it had some trouble with some states allowing sales. With Spyders, sidecar rigs, and trikes, the rider sits astride the machine and steers via handlebars, so that's kinda motorcycle-like. But you sit in the Slingshot and steer via a steering wheel -- more like a car. I already read about one legislator saying during the Slingshot dustup that "these three-wheelers are a growing category; we need to regulate them!"
We currently have it just where we want it. Let's not invite trouble. We may not like what happens if we get caught in the regulatory frag pattern.
[Edit -- Pleae note that I'm talking about street vehicles. Four-wheeled off-road toys don't count as cars.]