until I master 4 I do not want to add more work to the list!
No, this is the worse reply so far. Clearly you've never played anyting but a 4! If you had, you'd know that 5s and 6s are much less work than a 4. (Go see post #22 by Rev J the lazy bugger)
Personally I own 4, 5, 6, and 7 string basses. I play them ALL.
4. It's for old school classic bass lines that you want to keep authentic. It's especially important to keep me from "improving" classic basslines with extended range notes. But after playing 4 string for God knows how many years, I really don't enjoy the limitation anymore. 4 is great for slap too.
5. To me a 5 and a 6 are pretty much the same bass and I don't really care which one I play, but if I don't need the extended high range I'll drop back to 5 from a 6 just to reduce weight a bit. But usually the choice of 5 is because I don't need the high range and some of my favorite basses don't come in 6 string (Carvin AC50, G&L L2500 etc.) I did have the Carvin with high C for a while but then got lazy and went back to low B.
6. A six is essential for solos and any case (usually smaller groups) where that extended high end can work (chords etc.). I especially like 6 strings and 24 frets though it isn't a slap machine. One problem with 6 is that it looks bad in some bands. Blues Nazis have a fit over 6 string basses while 5 strings just fly right under the radar. But 6 is killer for blues solos so go figure.
7. I'm kind of a 7 string noob so I'm still getting used to what that last string is for. But it makes a lot of sense in cases as was mentioned where you want to carry melody and harmony (like when I sing). The other side of that coin is I have a 7 string guitar with extra heavy low strings tuned down so it can sort of cover bass as well as guitar. dig?
More than 7 strings (non-doubled - I do own an 8 string bass but it's doubled so it's really just a 4 on steroids) I find that with my size hands 7 strings is about my limit to comfortably play. If I went to more strings I'd have to totally change my approach going to tapping and the like.
I just choose the bass to fit the music. Simple as that. I have no problems going between them.