"It sees half as much impedance because it has to supply twice as much current for a given voltage"
Well if voltage is given but current is doubled then impedance should drop to the half - BUT it cannot be because speaker impedance does not depend on amplfier, weather and other extreneous factors.
In your own example - 16 volts across 4 Ohms - supply twice more current,4A, then 8 volts across THE SAME load, 4 Ohm i.e. 2A.
Just follow the Ohm law (V=IxR) and everything will be just fine.
Well if voltage is given but current is doubled then impedance should drop to the half - BUT it cannot be because speaker impedance does not depend on amplfier, weather and other extreneous factors.
In your own example - 16 volts across 4 Ohms - supply twice more current,4A, then 8 volts across THE SAME load, 4 Ohm i.e. 2A.
Just follow the Ohm law (V=IxR) and everything will be just fine.