Ahh...a four wheel drive buggy with a stepper on each wheel.
Can't answer your question without a few more questions.
How fast do you want it to go?
Geared or direct drive?
How large are the wheels (diameter)?
How steep of an incline? surface roughness? or a smooth, flat, level surface?
With my PIC test code I was running 4 steps in 100ms. I counted 10 steps in the motor so this works out to 240 RPM. The torque seemed to still be good so the motor could go faster with some PIC code changes.
If the wheels are 2cm diameter and are directly driven for the motor and the H-bridge delivered 3.0V to the coils then each wheel would push along the surface with a force of 0.012 Newtons.