How to get more torque using a Stepper motor
You have two possiblities.

1) You can increase the current in the motor. Your motors are rated for 4 amps. Use it. Be aware that this will increase the self-heating of the motors, and depending on your motor mounts you may overstress the motors. Using wood or plastic is not a good idea. Metal will conduct heat away from the motors much better.

2) Cut the number of microsteps. Using microsteps is a tradeoff of less torque for smoother operation and better resolution. You'll get maximum torque with no microstepping at all.