Many amps from the 80’s were rated not only into 4 Ohms but sometimes onto 2 Ohms as well. Big iron from Classe’, Krell, Levinson, Rowland, Threshold, etc.. Some even had more rated into lower impedances then than later on.
Vintage amplifier specifications
I've noticed when I read vintage amplifier specifications, and by vintage I mean 70's,80's maybe 90's, I do not see power output specified at 4 ohms. Did those older amplifiers not do well on 4 ohm speakers so leave that information out or was it not considered a a matter of interest to the buying public. Maybe something else?