Adbikes of the UK has a line of four wheelers with high cargo areas that can carry a billboard ad.
Eric has a four wheeled kiddie bike called the QQ-Bike that uses a rowing action to move forward and is steered by the feet. Feetz of the Netherlands has a four wheeled cargo and child carrier bike. It has a box for the kids in the front and another box for cargo in the rear.
Both models have electrical assistance as well as pedal power.
Alternative Vehicles has a line of four wheelers and trikes mostly made by Pacific Cycles in Asia. Berg Toys of The Netherlands has four wheelers that look like pedaled go-karts, and tricycles as well, including one with an articulated front wheel for body-lean steering called the Balanz Bike.
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.