Both models have electrical assistance as well as pedal power.
Adbikes of the UK has a line of four wheelers with high cargo areas that can carry a billboard ad.
They also have a 4 wheel courier and load carrying model.
You can contact the manufacturer for other dealers.
Contes Engineering has developed and will bring to market in 2012 an impressive four wheel machine that is intended for off-road riding and the kind of hard jumping that BMX riders do.
It will be pricey, of course, but if you want something for off-road that has a chance of holding up well, this one is worth a look. e Quadz from Concept Cycles in Montana, has a four wheeler with or without electric assist and fat tires for back road cycling.
The site has interesting info on their frame building techniques, as well as a detailed list of components.They have one model with a single bench seat and another with two seats. The Web site mentions drum brakes but does not mention gears.Carvx is a Dutch company with a four wheel mountain bike with four hydraulic brakes, full suspension and hub gearing.We could not find ordering or price info on the site, and the "news" page has items from 20, so it may never have made it into production.Chat 'n Bike in Gaithersburg, Maryland, is a US dealer for Dutch and Chinese four wheelers.They also have a four wheeler constructed of two side-by-side linked bikes for those who can ride in normal biking posture.