dc motor
I am new to electronics and robotics, so I apologize if my question is dumb.
I am trying to run four motors with two L293D, and here is the configuration that I am thinking of: 
DC Motor
This is the motor that I am using, but I am not sure how much voltage I need to provide to the motors via pin 8 of L293D. Do I just need to provide 3 – 6V? or do I need to consider other factors like how much current can the batteries handle?
Thanks in advance!
Googled "raspberry pi transistor" and found many results that would suit your need.
Make sure the transistor can handle the motors current.
PWM is possible with this configuration.
It is likely you can solve your problem by starting your motor slow then ramping up the speed. Tutorials are all over the web to do this. Here are three: 1, 2, 3.
To control motors the Arduino generates a fast pulsing signal that switches on and off over and over. The frequency is generally constant. However the duty cycle can vary. To make a motor go slow the on part is very small compared to the off part. This type of signal has a low duty cycle. Perhaps 0% to 25%. To make a motor go fast the on part is very large compared to the off part. This type of signal has a high duty cycle. Perhaps 75% to 100%. The following image shows duty cycles from 0% to 100%:
The Arduino function call analogWrite() is the secret to controlling the duty cycle of the Pulse Width Modulator (PWM) feature.
Best Answer
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).