Nema 14 with planetary gear 5:1 Rated Voltage?

I was looking for a stepper motor with high torque, and I found this NEMA 14 with planetary gear . The rated voltage is 4.2V. I have a A4988 driver but beacause its minimum operating voltage is 8V, I found this pololu DRV8834 low voltage driver. After two weeks it arrived and when connected the whole thing to my DC power supply and set the voltage to 4.2V with the basic code proposed in the HowToMechatronics website, the motor was only vibrating, instead of turning one cycle the one way, and two cycles the other way. I raised the voltage until the maximum rated voltage for the DRV8834, and still did not get the motor to rotate.
The I tried connecting it with the A4988, and when I raised the voltage over 12V it started turning, but the torque was not really promising. I started raising the voltage, and could feel how the planetary gear was starting to perform. At 21V and 1.2A, my capacitor across the power supply pins for the A9888 exploded.
I wonder:

-What does the rated voltage for this motor really mean?

The "rated voltage" is just the maximum coil current times the coil resistance and should be ignored. The important setting is the current limit, which is 1.5A, and that should not be exceeded.

For best performance, carefully set the current limit as high as the driver will allow and use the highest voltage allowed by the driver. The DRV8825 can handle 1.5A at up to 45V, but pay attention to all the warnings on the Pololu web pages!

my capacitor across the power supply pins for the A9888 exploded

What was its voltage rating? That IS important.

These links may be of interest.
Stepper Motor Basics
Simple Stepper Code

...R

Thanks a lot