Do you mean an amp that is rated only down to 4 ohms?
Those used to be "the standard". 2 ohm standard is only a last decade and half type of thing. Yeah there were older amps that could drive a lower impedance (DC300 for example), but they were rare and far between.
Some would drive a lower impedance-but with greatly reduced output.
For example a Peavey CS800 300 watt @8 ohm- 400 watts @ 4 ohm and 250 watts @ 2 ohm.
You could load them all the way down to 0 ohms and they would not break.
In fact the test for a repair was to short the outputs and drive the amp into full clip untill it overheated and shut down. Then remove the short and if the amp still works-you did a good repair. I did it many times.
Sorry, I couldn't find a sarcastic button, Ivan. A "4-ohm amp" ranks right up there with saying a speaker "pushes 300watts" with phrases I wish would disappear.