OK, this shield shouldn't have any problems with 9 VDC, but it's not intended for a total current of 4 A or more than 2 A going through a channel (in your case the motor only needs one channel). What was supplying the voltage; a bench power supply, a wall wart, or one or more batteries? It would be helpful to know the maximum current that the supply can provide.
However even without that information, I think the motor could have been drawing ~2 A (or possibly more than that).