What is the Difference between 4 ohm to 8 ohm output?

There is really no amplifier on the market which has an output impedance of 4 ohms or 8 ohms. The impedance of an amplifier is always less than 0.5 ohms. We do not use matching, we use bridging when we connect the amplifier to the loudspeaker. Scroll down to related links and look at "Interconnection of two audio units".