technical issue on paralleling 4 z-azis stepper motors

hello:

I am current plan to have DIY 3D printer that need 4 z-axis stepper motors.

I have tried traditional wire connection with mainboard with one single output step motor source, and parallel connecting two stepper motors on Z-axis,


However when same case happen but sharing four motors, it may not have enough current to run it, although from electronic theory that the voltage should remain the same, am I correct?

So my question is,
Does any hardware device that I can input extra electric power on this device in order to multiply the existing current, so that it have enough current to run 4 stepper motor?

Thanks,