Voltage required to run four motors
Your motors run on  3 to 6 V. 

But the L293 D has a minimum supply voltage of 4.5 V.

So you'll need between 4.5 and 6 V to power your motors.

If you want to be able to power all 4 motors at the same time, you'll need your supply to be able to provide a higher current than you would need to power a single motor. How much depends on how your  motors will be loaded. The current required is somwhere between 800 mA (4x the 200 mA no-load current) and 12 A (if you might overload and stall all 4 motors at the same time, however this might damage your L293D's, since they are only rated for 1.2 A per channel for brief periods).