Ideal supply voltage for stepper chopper driver
Don't be confused with voltage ratings described by resellers that they don't know what they are selling. For example the voltage rating for the motor you attached in link is 4V and it turns out that rated current 1.2A and rated resitance 3.3ohm needs those 4V (1.2 * 3.3 = 3.96V), but this voltage is needed at standstill, therefore no moving. When motor spins it produces back EMF and the voltage applied has to be greater in order to feed the motor with current. The real maximum operating voltage is indeed limited by the withstanding dielecric strength of insulating material.
Stepper motor driver is a current source and feeds the motor with constant current, the voltage is variable, rough formula V_driver = V_backEMF + R_wire * I_rated. V_backEMF = K[V/rpm] * N[rpm], not always the sellers know or share the most important parameter - K[V/rpm].
When decelerating, the motor turns into a generator mode and dumps the energy back into the caps. If caps have enough capacitance then the voltage rise is small, but if you have small caps then voltage can rise above max. permissible voltage that your driver can handle.