sjanes,
It depends on how much current the amp can supply. Once the amp reaches it's maximum current output capability, it's done. This is the most important specification in an amp, from a power only perspective (not considering the audio stuff like THD, etc.). You can calculate the maximum true power output if you know what the amp's maximum output current capability is.
To find the current, we first start with what we know. We know the resistance of the speaker and the max power output of the amp. We do not know what the current is at the max power setting. We can easily calculate it, though. Ohm's Law. Works every time.
power = current * voltage (P=IE)
We can't calculate it using that formula since there are 2 unknown values, voltage and current.
Voltage = current * resistance (E=IR), so we can substitute this in place of the currrent value in the power formula above.
power = current * (current * resistance) (or P = I*I*R, which is P=I^2R)
Doing a little algebra, we get current = the square root of power/resistance. ( I=SQRT(P/R) )
current = SQRT( 120W / 8 ohms) or the square root of 15, which is 3.8 amps. So, into an 8 ohm speaker, the amp will put out 3.8 amps. Now we know the voltage at the output the max current was measured at.
voltage = current * resistance (E=IR)
So, the voltage at max power is 3.8 amps * 8 ohms, which is 30.98 volts.
We can now determine what the current requirements at max power will be with a 4 ohm load.
current = voltage / resistance (I=E/R)
The current requirement of a 4 ohm load on a 30.98 volt source at max power will be 30.98 volts / 4 ohms, or 7.46 amps.
If the amp has a maximum current capability of 7.2 amps, it will "run out of gas" at 223 amps with a 4 ohm load.
That's why a high current amp is important. Current puts the "oomph" behind the voltage.
