You would steer the same way you would a treaded vehicle like a tank. To turn left, you would go forward with the right 2 wheels and in reverse with the left 2 wheels. It is less of a turn and more like a rotation.
Pauly:
In addition, if you want to do a gradual turn with a differentially steered robot chassis, you vary the speed of the treads by different amounts while they are going in the same direction. This can be done using the PWM speed selection mode of your controller for each motor.
What is really interesting about this platform though, is the fact that you have in actuality four independent "wheels" in a differential arrangement, but that are independently controlled; this is unlike a standard tank-tread differential steered vehicle, since such a vehicle only has two "wheels" to control (each tread). Perhaps by varying the speed and direction of the independent wheels, some other control/steering mode may be available? Sounds like something to experiment with!
Also - if you can actively brake the motors, that would be best, but those gearmotors might not backdrive well, so braking may be unnecessary.
