I've got some outdoor speakers that are rated at 8 ohms, and some car
speakers in the kitchen ceiling rated at 4 ohms.

A 30W Yamaha amp speaker outputs go through a computer-controlled
relay switch to direction the juice to either the kitchen or outside
depending on the setting.  I recently changed the switch to allow the
signal to go through to both kitchen AND outside. The result
(unsurprisingly) is that the music is too loud in the kitchen and too
quiet outside at any given volume.

So, I would like to retard the efficiency of the 4 ohm kitchen
speakers in some way so they behave like the 8ohm outdoor speakers ie.
increase their rating from 4 ohm to 8 ohm.  I have read a variety of
articles that sort of deal with this question, but I'd like to hear
from someone that really knows about this stuff before I go attaching
non-inducting resistors etc. to my 4 ohm speakers.

You should put some resistors in series with the 4 Ohm speakers, but I
can't predict how much resistance to use, because it depends on the
relative efficiency of the speakers and the acoustics of your indoor
and outdoor environments. You will also need to be sure the resistors
are rated to handle enough power and, as you say, their inductance at
audio frequency should be small enough not to affect the frequency
response on the high end. You could start with 4 Ohm power resistors,
but it would be trial and error. Why not buy one of those wall
mountable stereo speaker volume controls? They are rated by the number
of Watts you are trying to control. If you have a really powerful
amplifier, be sure you don't get an underrated volume control or you
could burn it out.

Thanks for advice - one option is definitely a manual volume control,
although they are pretty expensive and will be a real pain to fit (old
house). I might pick up some 4ohm resistors as you suggest and start a
trial. regards

Be sure to get resistors that can handle the power. If your amplifier
can deliver 30 Watts per channel into 8 ohms, you need to dissipate 15
Watts in each of the 4 Ohm resistors. To be safe, you should probably
use resistors rated for at least twice that much. Those are big
resistors.