After watching this video of an Animas Quadracycle, which is basically a four-wheeled bicycle, I think we should have more of these around. Not only can you seat 2 people safely balancing on all four wheels, the quadracycle does really well on rugged train and even snow where bicycles might have trouble.
If these quaracycles can be made with optional electric, it would possibly great commuters around town.
Clearly four wheels are required for car-like stability, hence the “Rhoades Car”, a pedal 4 wheeler produced for many years, but supple suspension and better brakes are required for off-road situations. Two person side-by-side seating, recumbent style, takes advantage of the 4 wheel layout. Independent freewheels for each rider, and a wide choice of gear ratios makes best use of the available human power.