Avoid voltage drop on DC motor start-up with h-bridge
Are you sure it's an undervoltage condition? I'd also check for a load short - note that in both cases the error flag is set and the voltage across the motor is zero. A 4-amp motor (under load) will draw much more than that on startup, and this may be causing the driver to shut down.

While connecting the motor directly to the battery will cause the motor to run, this may be simply because the battery provides enough current to get the motor going, which reduces the motor current, which allows the battery voltage to climb, which allows more current to the motor, etc.

What you need to do is get a scope and place it across the battery. Set the trigger to 6 volts, then connect the motor. If the scope doesn't trigger, you know that a 6-volt undervolt condition did not occur.