If you work with Arduino you will no doubt become familiar with two sites for resources. The first is the Arduino site itself which is at http://aduino.cc This is a very good site and is laid out well but the company has not put any R&D into a few boards that are mandatory for robot construction and masks its weaknesses in a mass of confusion leaving the casual experimenter empty.
One such product that the company lacks is a 4 channel or 4WD motor shield they do however offer a 2ch motor shield and back it with good code examples. This shield would be fine for allot of robot applications and this shield offers 1.2 amps of continuous motor current which is pretty substantial.
The second web resource that I would like to share is Adafruit at http://adafruit.com and they offer a motor shield that will drive 4 d.c. motors at 1.2A per motor with up to a 3A . See this offering at http://www.adafruit.com/products/1438 the code and libraries are different from the Arduino offering but with a little practice you should be up and running in no time. Please see my tutorial at 4 Channel Motor Coding.