Stepper Motor: how to drive and what voltage/current?

I have a stepper motor.
It has 6wires and looks like it may be a unipolar type.

The half coil resistance is ~4ohms. i.e. over the whole centre tapped coil I measure 8ohms. The two centre taps are isolated.

I have managed to get it turning using 4V +ve to the two centre taps and tapping -ve to the other four wires in sequence. It needs 48 pulses to turn one revolution, i.e. it is 7.5°/step.
I want to order a unipolar driver chip and many are rated <<1A and >>4V

Is there a way to determine what current and voltage I need to drive it?