I doubt they would implement a current limit scheme in such entry/mid level model. I think with 4ohm selector engaged it just drops the rail voltage. For example, if you half the maximum voltage, it would deliver the same current to a 4ohm resistor as it would at the normal voltage to an 8ohm resistor. You may say in efffect it limits the current but current limiting typically means it limits the current to a preset maximum value regardless of the impedance, including a short circuit, i.e. 0ohm. Current limiting can also get complicated if you have to allow for high short duration peaks. For an AVR, IMO it is actually better to lower the maximum voltage so if you have 4 ohm speaker you simply have the same or slighly less power and accept a 3dB drop in SPL (compared to 8ohm with the same sensitivity) but you don't have to worry about any current limits imposed by potentially not well designed current limiting scheme. Now I am only talking in terms of theory and guessing the scenario, I do not know exactly what Onkyo's 4/6 ohm selector does.