Impedance in your situation having 1 4 ohm and 1 8 ohm speaker leaves you in somewhat of a bind. Wiring them in parallel = 2.7 ohms, in series = 12 ohms. Ohms law to calculate total impedance in parallel is R1 X R2/R1+R2+etc. = Rt. In series the Rt = R1+R2+etc. Most amps will not handle this 2.7 ohms without causing series damage, as previously mentioned, and a 12 ohm load will require the amp to work harder to produce higher volume levels to the listner and probably produce distortion. Using the 4 ohm configuration is always the optimal wiring config. The amp runs cooler, almost idling, produces the highest quality of sound and with plenty of headroom to increase the volume. i.e. A 100W. amp @ 8 ohms produces 100W. However, a 100W. amp with a 4 ohm load increases the total Wattage to approximately 2/3 of the stated wattage power or approximately 166W. of total output power.
