Hello:
RMS Volts/resistance in OHMS = AMPS
Amps*RMS Volts = Watts
Given the voltage does not change, doubling the resistance will half the power. However this is rarely true because the power transformer voltage will sag when the load is increased. SO......... 100 watts into 4 ohms will most likely net around 60~65 watts into 8 ohms.
Example: SA21 nets 300 watts (per channel, both sides driven) @ 2ohms
200 watts (per channel, both sides driven) @ 4 ohms
125 watts (per channel, both sides driven) @ 8 ohms