Nach, this has been discussed here quite a few times, but as Andrew indicated, the 4 ohm setting shouldn't be used. It in no way optimizes performance, instead it simply reduces the voltage output of the power supply section to lower the chance of overheating and satisfy UL(Underwriters Laboratory)requirements. Following Ohm's Law, when the voltage is reduced, so is the current and power(and thus also the chance of overheating).
When lab tests have been done(e.g., S&V)use of the lower setting has cut maximum power roughly in half. So, overheating risk is reduced at a cost to maximum performance capability, and the setting should be left at the higher default.
