Jim, let me give it a try. The expression of Ohm's law which is probably most in point with Doc Jon's question is I=E/R(i.e. current equals voltage divided by resistance). This shows us that if the voltage isn't reduced, when the resistance of the speaker is 4ohms rather than 8ohms the current will be doubled.If the receiver can't handle this and is in danger of overheating its protective circuit will cause a shutdown. To prevent this many receivers have a "4 ohm" switch on the back or setting in the setup menu. This reduces the voltage which can be supplied and thereby the maximum current flow,without a shutdown. Since it also reduces the maximum power available this setting should only be used when experience has showed it to be absolutely necessary.
Then, despite knowing all the equations,sometimes mysterious shutdowns plague even powerful THX certified(which certification includes testing with low impedance speakers)receivers such as early samples of sushi's Pioneer 45TX. Apparently Pioneer has now corrected what may have been an overly-sensitive protective circuit and now just about anything goes.
