What you need is an ESC (Electronic Speed Controller) for each pair of motors, controlled by servo signals from the Tinkerforge Servo Brick. The two motors on each side of the robot platform should be connected in parallel so they operate together (there is no point having separate control because if the motors aren't going at the same speed in the same direction then the wheels will just skid).
Each ESC needs to have the following specs:-
Designed to operate brushed DC motors.
Controlled by hobby servo signal (1~2ms pulse every 20ms).
Bidirectional control without reverse braking function.
Rated for enough current to drive two of your motors.
This type of ESC is commonly used in RC cars and boats. Car ESC's usually have a brake function in reverse, which can often be turned off by programming the ESC for 'rock crawler' mode.
The ESC has power input wires for connecting to a dedicated motor battery. Many ESC's also have an on-board BEC (Battery Eliminator Circuit) which is a regulator that drops the motor battery voltage down to 5~6V to run a receiver and servos. To prevent this BEC from interfering with the Tinkerforge Servo Brick's own supply voltage, you must disconnect the center (red) wire from each ESC's servo plug.
Here is a suitable ESC that several vendors are selling on eBay:-
RC 10A Brushed ESC Motor Speed Controller No Brake For 1/16-24 Car Boat F05427