The early Beomasters (and the odd early Beolab) were rated for output into 4 ohms as that gave them a higher power output when comparing performance figures with competitors. Over time this became less important and most amplifier outputs and B&O speakers were changed to "8 ohms". The latter is more important when running mutiple sepakers for example on an early MCL 82 type link but otherwise there is not much difference - apart from the aforementioned power - ie with 8 ohms you will not get quite so much overall volume as you could with a lower impedance.
In other words it should not make too much difference - but if you plan to run multiple passive speakers start with 8 ohms!
Brian