My mistake, you're quite right. The total dissipation in the 0.4 ohm resistor will be 4v * 10A = 40W and this gets split between the three 1.2 ohm resistors. The power dissipation in the motor will also be around 40W in the stalled condition.
This demonstrates the advantage of active current limiting. The principle is to sense the current in the motor and adjust the PWM fast enough to keep the current below a set value, rather than use series resistors.