When I first took up bass, my mentor (old-school country, R&B guy) advised me that what a bassist doesn't do is more important than what she or he does.
For instance: doesn't slap, doesn't play chords, doesn't over-play;
does play flats and only four of them.
When I told him early on that I was considering a 5 string "because it would be nice to have one more D and C" he just shook his head slowly, as if a close relative had died.
I never found a 5 that felt right (either scale too long or strings too floppy). Also, I could never get the feeling that I had as complete control over the tone, timing, accents, etc. as with a good 4.
And like my mentor said when he noticed that I decided to stick with 4's, "there are still plenty of low notes on a four string."
It's inconceivable to me now that I'll ever switch.