OK, so the voltage is not the direct voltage on the motor winding, but the calculated PWM mean voltage value based on the duty-cycle of the PWM used - right?
Thank you for clairyfying this to me, because I am not used to work with stepper motors. I had the frequencies and number of pole pairs wrong by a factor four in my previous calculations above. 1.8 degrees in data sheet means 1.8 degree per full step. And four full steps make an electrical cycle. So at 4000 rpm you should expect a reactance of the winding of 77 ohm.
This is a link to the datasheet of a similar stepper motor, and they show how they expect the moment to drop at increasing speed:
https://www.faulhaber.com/fileadmin/Import/Media/EN_DM66200H_FPS.pdf