Dave, one form of Ohm's law is that current(in amps) is equal to voltage(in volts)divided by resistance(in ohms), i.e. I=V/R. So, if the voltage delivered remains constant, but the resistance(or impedance)of the speaker is 4 ohms instead of 8 ohms, the current is doubled. Some amps might not be able to handle the increased current, would overheat, which would start a fire, which would burn down the house, which would burn down the rest of the city, etc.(if the protective circuit in the amp didn't simply shut it off). A switch or setting for 4 ohms doesn't really solve a problem if it would exist, but just covers it up. The switch reduces the voltage available and therefore the current, even if the speaker is of 4 ohms impedance. Since this also reduces the power available(power in watts equals current in amps times voltage in volts, i.e. P=IxV)it should only be used as a last resort if the amplifier keeps shutting off. The majority of receivers can handle the situation, but each combination of receiver, speakers and sound level is a different case.
