The 4A Motor Shield is based on the Freescale MC33932 dual H-Bridge Power IC , which can control inductive loads with currents up to 5.0A peak per single bridge. It lets you drive two DC motors with your Arduino/Seeeduino board, controlling the speed and direction of each one independently. You can also measure the motor current absorption of each motor, among other features.
The on board DC/DC converter suport a very wide range of input voltage, and can provide a 5V power supply for the microcontroller board with 100mA maximun current.So, you just need one power supply to drive the motors and power up the logic circuit.
Features

Output short-circuit protection (short to VPWR or GND)
Over-current limiting (regulation) via internal constant-off-time PWM
Temperature dependant current limit threshold reduction

Arduino 4A Motor Shield Pinouts