I'm not very familiar with their shields, or shields in general, but in the video on their webpage they said you can stack them and that allows you the addition of 2 more motors...not 4 more. Read the info they provide first. It will probably answer more questions than I'm able to. The arduino's analog outputs are ideal for motors, but you can do it with the digital ones via pwm. I did a quick search and found this, Arduino Playground - DCMotorControl. That might get you started. There is a vast amount of information on this. I highly encourage you to search around this website and google what you're doing here. It's a very common topic. Also, once you have the parts and get to start putting things together it will make much more sense.