Use a power mosfet, either a P-channel one switching the positive supply to the motors as in your diagram, or an N-channel mosfet switching the negative side - which is easier to drive and gives you a better choice of mosfets.
However, I suspect that you need to control the speed of your 4 motors individually. In that case, you can combine the on/off function with the motor speed control, avoiding the need for a separate electrically-controlled on/off switch (you'll probably still want a mechanical on/off switch for safety).