The best I see is deriving a theoretical torque and that number would not even take motor inefficiency, drag and other factors into consideration. At that you would still need the rotational speed of the motor.
18 Volts * 4 Amps = 72 Watts
72 Watts / 746 Watts (1 HP) = .0965 HP
Divide the HP / RPM (Unknown)
Multiply by a constant of 63,025 (the answer will be expressed in LbFt of torque)
Even with the RPM known for the current and voltage the answer would be theoretical and likely not even close to the real world torque the motor was actually producing. Also, I am not even sure how correct what I stated was. I think it goes that way and am also unsure of the constant I used.
Maybe another member has a better solution?