Amp power output in relation to impedance change

How does an amplifier’s power output change in relation to impedance delta? Is it a continuous change or does it wait for certain thresholds?

for example:

Amp1 is rated at 200W@8Ohm and 400W@4Ohm. When the speaker creates let’s say a 5Ohm impedance, how much will the power output be in that case?

Amp2 is 250W@8Ohm and 300W@4Ohm, how much will the amp output be at 5Ohm?

Is the answer the same for all the classes? (A,AB and D) for example.

thank you!