Ooh, I see potential for some heated discussions here.
For me 4 holds the essence of what a bass guitar should be, it sits and fits perfectly and is the bass I feel I have most freedom on. 5 or more gives you extra range and more girth to your tone but for me it does come at the expense of some freedom as technique needs to be a little cleaner on my 5.
I could not do without either my 4 or my 5, both provide different things and require a different approach to bass playing. But none is better or worse, just different.
I used to have a guitar player that felt 4 was too simple and had the warped opinion that proficiency had some correlation with the amount of strings on your bass......... Wooten, Pastorius, Jamerson, McCartney, Flea all feel 4 is enough. And I have seen more than my fair share of bass players show up with 5,6 or even 7 string basses who would struggle with a 4.