Its not as simple as just 4 ohms verses 8 ohms. There are other variables to consider such as phase angle and the frequency at which the 4 ohms starts to present itself. With a small phase angle, a 4 ohm load could be much easier to drive than a 8 ohm load with a large phase angle. Similarily, if teh 4 ohm is above the mid bass, then most quality AVRs will be able to drive a 4 ohm load. Also, if a speaker's senstivity is fairly high and the room is medium to small, then most quality AVRs can drive a 4 ohm speaker. Its more complex than just 4 verses 8 ohms
Just to clarify and add to. Impedance is made of two components Resistance and Capacitance/Inductance combined together in a pythagorean triangle. impedence is the vector sum of reactance ( inductance or capacitance) and resistance.
Xc = Capacitve Reactance= -1/(2 * Pi * f * C) so C reactance diminishes as the frequency increases. It lags in phase.
Xl = Inductive Reactance = 2 * Pi *f * L so L reactance increases with frequency
It leads phase.
Impedance = Z = sqrt ( R^2 + (Xl-Xc)^2 )
Resonance is the condition that occurs when the inductive
reactance and capacitive reactance in a circuit are equal.
When this happens, the two reactances cancel each other,
leaving the circuit with no impedance except for whatever dc
resistance exists in the circuit. Thus, very large currents are
possible in resonant circuits.
The entire concept of phase angle changes with frequency and the highest current occurs at resonant frequency. Fr = 1/(2*Pi* Sqrt(LC) ).
Phase angle and frequency are directly related.
Good Luck!
Forest Man