If the extra 2 wheels are removable, like a Voyager kit, then it is indeed legal, as a 2 wheeled scooter, since the extra wheels are not a permanent part of it's design. But as far as a 4 wheeled motorcycle, it seems like a quad might fit that description, and I had a street legal quad for about 3 years. It did not come from the factory that way, but all I had to do was put a mirror, horn, and license plate light on it (it already had head, tail, and brake lights) and they gave me a street legal title for it. It cost me less than $100 a year to insure it. There are a lot of street legal quads and dirt bikes in my state, it is easy to do, because there really is no such thing as off road here, unless it is on private property. If it is on public property, then it must be street legal.
The Voyager kit is popular among Goldwing riders, and I've never heard of anyone having any legal problems with them. They are considered more of an outrigger, kind of like training wheels on a kids bike. You do have to have an actual motorcycle license to ride a vehicle like that, while a true trike only requires a drivers license.