I think they are DC motors judging by the code on my page here:http://www.gammon.com.au/forum/?id=11506
They are driven by the motor board with PWM output (and a forwards/reverse signal).
I don't think 4 motors would be faster (each one would have the same maximum speed) but might be able to go up higher hills. Although I found mine with 2 motors was powerful enough to flip itself onto its back.
You might want to consider as an alternative the Parallax BOE Bot:http://learn.parallax.com/node/129
That was completely self-contained (apart from the Arduino), and also came with parts, and instructions, for an IR sensor, and light detection. For example, this page:http://learn.parallax.com/ShieldRobot/LightSensors
I found the Parallax BOE Bot was easy to assemble, and the parts fitted together well. Plus the instructions on the site above are very comprehensive and detailed.
However you should be able to do the same with the Dagu, except you may find it a bit fiddlier to assemble, and you'll need to get the light sensor and other parts yourself.