Arduino, L298N and 4x 5 V motors voltage loss

[yamadanao]

I'm building a circuit with an Arduino Uno rev3, a 12 V wall adapter connected to the Arduino power port, 4x 5 v 180 mA motors and an L298N motor controller. Here is the date sheet of l298n:http://www.componentschip.com/details/STMicroelectronics/L298N.html

The L298N controller is powered from the Arduino 5 V out pin. When I connect only two motors, everything is fine, but when I connect four and power them, there is a voltage loss and the motors start to work slower to shut down completely. What voltage do I have to supply to the controller to be enough for powering the 4 motors at once and (possibly) can you give a link to any AC adapter suitable for powering it (the adapter has to have two output + and - cables, not the circular wire like for powering the Arduino)?