I'm with John on this one. I changed to 5 string in 1995 and have very rarely even touched a 4 string since (george's series 1 being about the only exception). I don't feel comfortable playing a 4 string nowadays, finding the neck feels a bit like a toothpick. That said, I still have both my 4 strings and am unlikely to ever part with them but they're currently in the loft gathering dust.
To slightly hijack the discussion and without wishing to appear insulting in any way, I do find comments like Terry's above...Four was good enough for Jaco, Jamerson, Clarke et al, it's good enough for me ... to be a bit odd.
Firstly, whilst I admire all the above players, I'd like to think that we're all trying to advance what is after all, a very young instrument. Stating that it's 'good enough' suggests stagnation and I don't think any of the aforementioned players would see that as a good thing although in my own opinion (which will no doubt differ from a great many others)' Stanley hasn't done anything 'new' for years. (don't get me wrong, I love his music but the last few albums sound so much alike to me that I rarely listen to them)
Also, Jaco was a pioneer in the use of effects and loops for bass. I'd like to think that, had he lived, he'd have embraced each advance in the instrument if not actually leading the way. Who knows?, he might even have taken up the chapman stick or extended range instruments.
In short, I want to develop my own musical voice and I've found a five string works best for me. Some have the same feelings, some prefer to stick with a 4. I find it interesting that the bassists that get most exposure in the musical press, for instance marcus and victor, tend to use whatever instrument works for the song and in both these cases they frequently stray into 5 string territory.
Graeme (rambling slightly - it's the unexpected heat)