Guidelines for current draw of stepper motors vs Drivers vs Power supply

1. It's a 4.2Amp motor.
2. Steppers use the most current and are strongest while at a standstill. Both current and force reduce with increase in rotation. They have different characteristics which can be somewhat predicted if you can get a hold of a torque curve chart. These characteristics can be improved to an extent with better drivers. Steppers are easier to stall the faster they go. Some drivers are better than others. Some have a half current setting to reduce heating while the motors are not moving.
3. The current limit pots on the gshield will determine what current gets to the motor.

In my opinion this is not a good motor and driver combination. It's akin to putting substandard fuel in a high performance vehicle. You'll likely experience crunchy motors and hot drivers. You may want to consider another higher capacity driver or swapping those motors out for something with less current requirement. With drivers it's best to have a little more than you need for the motors, but the DQ542ma's in the storeroom would probably work very well for those guys.
Just my 2c

Here is a nice article that goes through the ins and outs of stepper motor and driver ratings and describes how the term phase is used regarding them; http://machinedesign.com/archive/stepper-phase-current-made-easy

Joe