Unfortunately, we do not know what modern amps
will do at 4 ohms. If their power supplies and
heat sinks were really beefy, they would put out
exactly twice the power at 4 ohms as at 8 ohms.
Some of the early RMS-rated amps did exactly
that. But some would put out, say, 50 watts at
8 ohms and 75 watts at 4 ohms, indicating that
there was some lack in their power supplies or
that the heat generated in the amp with a 4 ohm
load would not sustain more than 75 watts. Today,
who knows? It is a pretty safe bet that they
will put out a bit LESS at 4 ohms than at 8 ohms,
and I am being conservative; they will probably
put out more. But since they are not measured
that way, we cannot know.