Not the brand, the type... e.g. Bipolar, Unipolar, and you also need to think about the mode, Full step, half step, microstep... It's different for each. The page at:
techref.massmind.org/techref/io/stepper/power.htm runs down all the options and is kept up to date.
Worst case: You need double the rated current per motor. So for a 2 amp motor, supply 4 amps, and for 4 of those, a total of 16 amps. Depending on the driver and mode, you may need as little as 8 amps... but then you will need a higher voltage and you will NOT get the full torque possible.