4 ohms to 8 ohm speakers?

Of course the tap that sounds the best to you is the one to use, but the one that sounds the loudest indicates the most efficient power transfer, and potentially lowest distortion. But this factor will be all over the map with different speaker/amp combos regardless of nominal ratings, so experimentation is the only way.

For some reason the link I provided doesn't seem to work, at least on my PC.

The thread name is "Dunlavy 4 or 8 ohms for Audio Research VT-100 amp?". You can do a search for it.

I'm starting to believe 16 ohm speaker drivers are better! The system they are used in may be 8 ohms, or even 4 ohms, based on how many are in parallel. But the real key might not be the impedance at all, but instead the NUMBER OF TURNS IN THE GAP. Mutual impedance of turns cause the motor force per ampere (BL) to go up with turn density. As thinner wire is the only way to get a higher voice coil impedance of the same winding length, you can get more turns either next to each other, or in additional layers in the same gap width/height as well. I'm doing some real-world investigations on whether or not drivers from the same family (cone, suspension, magnet, frame) sound and measure differently based on turn density. Also, your amp gets to be lazy!

:)