Unused slots in the stepper Motor

I am wondering if all of the specs need to be matched or are there some that don't need to be exact.

It completely depends on the application. Unfortunately you don't give us enough information to provide you with a good answer. However, in general stepper motors are fairly standardized. If you get one with the same step size, and the same general shape and size, and you can wire it up correctly (since the driver uses 4 wires it's pretty easy to convert a 6 or 8 wire to 4 wire use) then you should be fine.

If the device that uses the stepper is using it near its limits in terms of speed, torque, etc then you may have problems if you don't match the winding impedance, mechanical resonance, etc. This is only a problem in high performance machines, though.

I would contact the manufacturer, if possible, and get their recommendation. If not, try a different motor out and see if it works. Chances are good it'll be fine.

The step size is independent of the wiring - a four wire stepper at 0.9 degrees per step is the same as a 6 wire and 8 wire, just the coils have taps in them (or separated). So you will need to match the step size regardless of the wiring.

I would suggest driving the motor with 72% of rated current, or about 310mA. That should yield the same power dissipation as would be expected from unipolar drive at 440mA. When the motor isn't moving, the voltage would have to be about 31 volts. I wouldn't expect insulation problems at 31 volts, or even 50, but I don't know how far one could safely go. Perhaps measure the AC voltage when hand-spinning the motor quickly to see if there seems to be a limit.