It needs a current limiting circuit.
That can be as simple as a series power resistor to set the current.
That wastes a lot of power and heat though, so most modern stepper motor driver ICs use PWM to continuously adjust the current and keep it to whatever has been set for that specific motor.
The motor coils are inductors. When power is switched on to a coil/inductor, the current starts at zero and builds up at some as the magnetic field increases.
Running at 4V would only allow the motor to run very slowly, as the current takes significant time to build at each step.
Using a higher voltage and a current limiting circuit makes the current built much faster and allows the motor to operate correctly.
The faster you need the motor to run, the more important that is, as it has less time between steps so without the higher voltage the torque will rapidly drop to zero as you try to increase the speed.
It drops off with speed regardless, but at much higher speeds with a suitable voltage and drive circuits.