Arduino + simple DC motors? (4WD kit)
First you make 4 parallel H-Bridges, http://en.wikipedia.org/wiki/H_bridge then you attach it to an external powersource ( or attach directly to the battery to prevent burning your arduino )

Attach every up to a pin, making it high makes the motor running, you need 8 pins for this, eg pin 1 makes motor 1 go forward, pin 2 makes motor 1 go back and so.

This is by far the most practical solution, but it is the easiest, and can be done with 8 PNP and 8 NPN transistors.

If you don't know how transistors work i suggest https://www.youtube.com/watch?v=-td7YT-Pums collins lab, it is both educational and entertaining.