Today is back to basics day.
“If you have an amplifier that puts a certain wattage out at 4 ohms, do you need a speaker that is rated at 4 ohms or doesn’t it matter?”
It doesn’t need to be 4 ohms, but it does matter what the impedance is. The impedance of the speaker(s) determines the “load” that is presented to the output of the power amp. The lower the impedance the more current will flow (see Ohm’s Law), which translates to more power (wattage). An amp that may put out 100 watts into 8 ohms may put out anywhere from 150 to almost 200 watts into 4 ohms. The impedance of the speaker just gives you a way to know how you are loading your amp. In the case of some guitar amps, they have switches you must set to align them to the load impedance for optimal performance. The main thing you want to watch out for in all of this is not to put too great a load on your amp. Lower impedances are greater loads. At 2 ohms your amp has to work pretty hard. Some amps can do it without problem, while others may fail. When you connect multiple speakers in parallel to your amp you are lowering the load impedance according to the formula presented in the 8/24/99 issue of inSync.