If you want acoustics, there are acoustic basses. Be prepared to start counting at 300 euros. I got mine for 200, but that was just really lucky in getting a good sounding one. I tried some worth 400, and those sounded worse then the case you'd carry them in.
Anyway, I've only played 4 strings, and I've rarely missed anything, except a lower string in one or two Counting Crows songs, but their bassist must love odd stuff, since he always uses odd tunings anyway.
4 strings is plenty for most baselines, but playing bass can indeed get a bit boring unless you have either a very nice bass line, or like listening to a melody while you play.
Do you play another instrument already?