How to interpret the stepper motor voltage requirement?
There are 2 approaches to driving a stepper motor.

The simplest is just to connect DC to each winding in turn, via switches (FETs, driver ICs). And in that case, use 4.8V (5V - switch losses) as you confirmed from current and resistance. This is fine at low and medium speeds.

If you need maximum performance, you'll find the motor's inductance attenuates short pulses, so running the motor faster reduces its torque. You can overcome this with a more complex stepper driver, supplying pulses at the recommended 12-24V, to maintain current and torque at higher speeds. 

Each pulse is maintained at a high voltage for long enough to build the rated current in the phase, then it should reduce in voltage to the safe level of 4.8V for the remainder of a slow pulse or steady state. This reduction in voltage can either be timed, or achieved by monitoring and limiting the drive current.

So both voltage ratings can be correct : 4.8V continuous, and 12-24V for an optional boost to high speed performance.