I truly only NEED 4, but I like having options.
I need 5. I love being able to walk up from the low D to G. I rarely have an opportunity to hold a sustained note below E, but on the occasions when I do it's good fun. There are tons of great 5's at reasonable prices, or at ridiculous prices if you want to go that route.
I have played many 6's, and I find that often the C string sounds a bit thin. There are exceptions, such as the Roscoe 6 I played at a recent TB GTG... but all of the 6's I've played that really floated my boat were pretty pricey. The high string would be nice for soloing, but perhaps even more so for adding some high chordal bits and for opening up new harmonics. I've been put off by prices, and by the fact that a lot of 6's have wide string spacing and therefore wide necks that I worry would be fatiguing over the course of a 4-hour gig.
So, play some of each. You will certainly not regret going to a 5... once you get comfortable not only with the "low notes" but with exploring new positions on the neck, you will likely be hooked for life. I can still play a 4, but find myself constrained without that B string.
