[Wanders in, reviews original post.... Thinks to self "Hmmm..., I ought to know something about multi-wheeled human-powered vehicles...." Looks at his own avatar....]
Ah! Tips and tricks for a four-wheeled pedal-vehicle.
Google "Ackerman principle" and study until you understand it. In a nut shell, the front wheels must follow two different curves in a turn. If they stay parallell when they turn, the rolling resistance will be fierce.
I don't know how many people you plan to have pedaling, but make sure each rider has his own freewheel. Otherwise they have to pedal in perfect sync, which ain't realistic.
On the Atomic Zombie forum, there is some guy who sells adapters to mount a bicycle freewheel on a 3/4 inch shaft. That guy is me. I pass them along at my cost, mostly to my buddies in Kinetic Sculpture Racing, but happily to any other pedal-vehicle builder also. I'm almost out of them, but will eventually have another batch made. (No other connection with Atomic Zombie.)
I have seen several of the American Speedster kit vehicles in action, and they tend to have trouble with the chain falling off. Wobbly chain ring, crooked chain line, flexible mountings, etc. Still, there are a number of them in service on the Playa -- just plan on doing some improvements to the basic kit to make it work.
With a four wheeler, the issue of a suspension can be an... uh... issue. Essentially, on a flat surface, a four wheel vehicle needs no suspension. But when the terrain is uneven, a four-wheeler will be teetering on two or three wheels most of the time. In Black Rock City, this isn't much of a problem, but you should be aware of how it works. For example, you would not want to have most of your weight on the undriven wheels, as a driven wheel would then be unloaded -- and would spin -- when you ride over an uneven spot.
Many three- and four-wheeled pedal-vehicles have drive on only one wheel. This can work fine. But if you have at least two people on the vehicle, there is no reason not to drive two wheels -- which will give you much more positive propulsion in the occational spot of soft ground.
Do not be tempted to drive two wheels on a solid axle. Such a vehicle is impossible to steer on pavement, and not real easy to steer even on the playa.
I avoid differentials myself, but if you "must" use one, you can find one cheap under many an old lawn tractor / riding lawn mower. Be sure to use four bearings -- two near the diff and two near the wheels -- all nicely lined up. Otherwise, the diff WILL eventually break.
That help any?