Most of the junior high school students I work with seem to learn best when they don't have to shift their left-hand position while they're learning to read, understand rhythms, etc. When asked to shift their left-hand position (especially above the fifth fret), their concentration is broken, and their time falters.
Were I starting over today, I'd probably begin with a 5-string with E-A-D-G-C tuning because it would be far easier to concentrate on reading and rhythm without the distraction of having to move the left hand unnecessarily; once the basics of musicianship are firmly in place, it becomes far easier to explore the upper reaches of the fingerboard.
But, hey, I'm an old fart:YMMV
It depends. If you know you'd like to end up playing a 5, then go ahead and get one, because it's not that huge of a jump from a 4.
My situation was a bit different. I had the idea that I wanted to play a 6 string fretless, but I started out on a fretted 4 string so that I could get my technique down without having to worry about intonation or tripping over extra strings. I became ready for the fretless pretty quickly, and I spent a long time wishing I had mwah and extra strings, but I think that I may have been ready for the fretless 6 so quickly because I kept things simple to start with, whereas buying a 6 from the beginning might have been a little too much for me to handle.
Both of you make great points. I'd add this:
Yes, 4 string is easier. Anything you learn on 4-string will apply to 5-string. However, those who say about 5 string that "it's only an extra 5-lower semi-tones, or your can just ignore the 5th string if you don't want it," may have over-looked a couple of important things.
As many 5 string players know, fingerings on 5-string CAN and in many cases SHOULD be different from 4 string. This is the main advantage of 5-string, IMHO.
Thus, IMHO, it is not the lower-register that makes 5-string so different and so important. Rather, it is the availability of notes a 4th lower than those on the E string for any given position, i.e., without shifting your fretting/fingering hand. This is assuming BEADG.
BTW, I tune one of my 5ers to EADGC, and find that tuning very very flexible and useful for solo work. Again, you can reach things without as much shifting, as Jazzdog noted.
I think that modern bassists need to be able to handle 4, 5, and maybe six strings, on fretted and fretless fingerboards. Ideally, we should all be good on 4-string upright, too, including the use of the bow. There is nothing like arco playing to build left-hand technique for intonation, and this definately (IMHO) supports your fretless playing in several ways.
Which bass to start on? hmmnnn....I started in the childrens choir, took piano, then french horn. I then got a bass (4-string electric, fretted), and then started upright. All this before I was 17. Start early and start often, I always say!
But seriously though, just get a bass and do it, NOW! I don't think it matters that much, but 4-string is easier. Fewer barriers in the beginning of anything is good. Lord knows there are already so many things trying to keep us as worker/consumer-bees, and away from making music. Art is just way way too subversive!
As you say, might as well start on 5.
) I always want to go down to lower notes from my rote and started detuning.
It's better to learn the right technique to start with, rather than delaying it until later in your learning curve. 
I saw someone selling Warwick Corvette but he says there are some cracks on the neck which scares me.. an Ibanez K5 but some ppl say the gap between strings are too narrow and I don't know if I can bore that 'K5' logo when I play at church.. haha
