I am not sure of 4 dc motors but i used a much simpler way to drive with 2 dc motors using the same L293 D H-bridge and the same principle might give you an idea.
In my case, the left two wheels are connected to a dc motor and the right two wheels are connected to another.
If you exite the left dc motor and turn off the power supply to the right dc motor, the motor will steer right based on its inertia.
If you exite the right dc motor and turn off the power supply to the left dc motor, the motor will steer left.
Exite both the motors in the same direction forward - the bot goes forward and exite in the opposite direction to move backwards.
You might use the same signal for your left two dc motors and another signal to control right two dc motors as in my case and the duration of the excitation and turn everything is just trial and error. Start with this and you ll end up with various ideas.
Hope this helps !!!