The nominal rating is the impedance the speaker manufacturer wants you to think the speaker is. The minimum impedance is closer to what really matters: what's the toughest load a speaker presents to an amplifier? (Impedance varies with frequency, as you may already know.) But when I see a minimum impedance of 4 ohms, I generally assume that means "3 ohms."
As for amps, it's not so easy. What you'd like to know (among other things) is how much power the amp can produce in a short burst at the frequency at which your speaker's impedance dips to its lowest point. But your power spec is for a continuous tone (by government fiat, by the way), usually at 8 ohms.
As James Boyk once wrote, the only two specs that mean anything are the dimensions and the weight. The dimensions tell you whether it will fit on your shelf, and the weight tells you if your shelf can hold it.
As for amps, it's not so easy. What you'd like to know (among other things) is how much power the amp can produce in a short burst at the frequency at which your speaker's impedance dips to its lowest point. But your power spec is for a continuous tone (by government fiat, by the way), usually at 8 ohms.
As James Boyk once wrote, the only two specs that mean anything are the dimensions and the weight. The dimensions tell you whether it will fit on your shelf, and the weight tells you if your shelf can hold it.