> Well, the major pentatonic scale does not include the fourth; the
> locrian scale does not include the fifth; the Iwato (Japanese) scale
> does not include the fifth; the whole tone scale does not include the
> fourth or the fifth.
We're talking about two different things. You're talking about modes here, about subgroups of a scale. It's true they are often called scales. But clearly they all by definition leave out notes. And you don't have to go that far to find something without the fifth; you can find it the common and overused blues scale.
But I'm talking about the complete set of tones that divide up and constitute the entirety of an octave, like the pentatonic, the diatonic, the chromatic etc -- all of which include the fourth, fifth and octave.
And I believe that is true of the Indian and Arab scales even though they divide the octave into more than 12 units. For one thing, the Harmonic Minor, which includes the fourth and fifth of its corresponding major scale, is sometimes called the Mohameddan scale because it corresponds to a widely used Arabic maqam. So while there are more notes in the Arabic scale (which comes out as Ivesian quarter tones when you render them on Western instruments), it seems these two tones have to be in the complete set.
As for not matching the Western 12, you have to take into account that all scales with more five notes have to be tempered if played in a group with multiple instruments with a range of more than an octave unless you want to really limit your harmonic possibilities. If you interpret Pythagoras' ratios strictly, then no system in the world corresponds to it beyond the octaves, including ours. But if you if accept that as a basis to which all cultures adopt one of many possible solutions to the tempering problem, then they all do, AFAIK. And the math was independently discovered in many different areas of the world, and arguably earlier in China than in Greece. Most likely in all areas it was originally discovered by comparing string lengths, where the octave ratio is kind of obvious if you're using a single kind of string.
Michael