Actually, I think the sax comparison is VERY apt! I see soprano, alto, and tenor saxes all quite regularly, why can't it be the same for bass?
That's because the different saxes each have a completely different timbral voice; and I do mean
completely. Also, a sax has a very limited range: only about 2.5 octaves!
(Let's not talk about altissimo here, it's a whole 'nuther can of worms.)
As for the bass debate, 4's vs. 5's, there's lots of great points, and figures, and passion, on both sides. I'm just going to say one thing though, and that is that the Fender Precision Bass has been in
continuous production for what, sixty years now? And how long has the Ampeg SVT been around, in more-or-less its original format?
What I'm getting at is, is that some of us (a large proportion, evidently) are just as much creatures of habit as, say, a classical violinist; so while there will always be crazy folks out there on the edge with 20-bazillion string fanned-fret electric violins they program to be played by robots, somebody somewhere is always going to be sawing on the the four gut strings with the old horsehair.
And similarly, no matter how many dudes swear by fives and credit-card sized SS amps, there will always be cats playin' it cool with a four-string and a tube amp.
Don't believe me? We have proof right here!
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