First of all, not even the string bass started with 4 strings. They just got frequently made with 4 (AND 5) strings after XIXth century. So, this is simply a matter of technological achievement summed up to some musician's desire to expand his voice range.
If you listen to lower pitchs in your head you shouldn't be limited to the low E, just this. Altough, despite my claims to fellow band mates that 6 strings are simply a broader bass, I have to addmit the Instrument has some unique features.
Seems that the 6 string is somehow more related to Acoustic Guitar. It's not just the extended range, the Instrument takes you to a more vertical way of thinking with all those strings. The 6 strings begs you to approachs it with an eye on the Bass, shure, but the other on the classical guitar.
I know this is just a single way to view it (not original and not for everyone's taste), but I really want to explore it this way. (As a brazilian) I started to study Villa-Lobos classical guitar stuff and I'm trying to apply it to the Bass.
I believe you can think about it as an entirely new Instrument, made to express other needs. It just has to create its own space, inviting musicians to explore its singularities and justify its use, as happen to the Eletric when it was born. Back then, people complained about it saying the Instrument was more of a Guitar. Yeah, may be... But what is the problem? Why an new Instrument should be taken as a threat?
I always tought the Bass as a function in Music. Althought it will affect your phrasing, it really doesn't matter if is a Tuba or an Organ's Pedal (or a Bass) you'll use to embelish the lower portion of a Chord. Witch one you'll pick is a matter of affection.
I also like more the 4 string, but I don't think it really makes all things the 6 does. If the Eletric deserves a space by its own, I don't know how accepting the 6 should be a problem.
(Message edited by Mario Farufyno on December 12, 2008)