I vote go with 5- IMO and IME the advantages far outweigh the disadvantages- you can transpose tunes, you can play with guitarists who tune down without bringing a different bass, and you can do all the stuff you can do on a normal 4! I'm currently wielding a 7 string (bass) as my main which may be overkill but if I was to use anything else it would be a 5. I wouldn't feel prepared with just 4.