Late to the thread, probably adding redundant comments, but here goes anyway.
I played four string exclusively starting around 1975. Naturally, I felt the need for lower notes, and fairly quickly adopted the "D-Tuner", now the Hipshot Bass Xtender. It's a great device, I put them on all my four strings, learning how to transpose the low string, but it wasn't quite enough for my needs so in 2000 I finally decided to give 5-string bass a try.
Long story short, I now play 5-string exclusively; I no longer own any fours. I don't recommend abandoning four -- just sayin' that's how it worked out for me.
I was gigging on 5-string after a month, but it took me 2-3 months to get comfortable on it. One problem was knowing which string was which, another was keeping strings muted.
Here are a couple of things I did that helped:
- I put my four strings aside and immersed myself in five string
- Playing the same ol' songs on five string led to confusion. As it happens, I joined a new band in Y2K, and had to learn a whole bunch of new (for me) cover songs. I decided to do that on my five string. This helped me a lot: no "muscle memory" to throw me off; I was able to map them out fresh on my five string
- I adopted "floating thumb" to mute the extra strings. I still can't say I've fully adopted it, but for some songs it's necessary
Once I became comfortable on five string, I realized all the advantages. It isn't just for the low notes (though it is great to do stuff like walk up to low E) -- it helps a lot even on songs on which I never travel below low E. I remapped numerous songs I'd played on four to take advantage of the additional range in each fingering position. That's basically why I decided to abandon four string, though again, I don't recommend that anyone else do that.
One final tip: don't be wary of the B string, or think of 5-string as "EADG plus one". Grok the entire instrument, keeping in mind that the B string is useful above the 5th fret, where it's in 4-string territory. Sure, you don't want to abuse those sub-E notes, but OTOH, don't be afraid to add them to classic/oldies tunes that don't go below low E -- sometimes they work beautifully.