I played guitar for about 15 years before adding bass to the mix.
I got a 4 because basses have 4 strings. If I want 6 strings, I’ll pick up a guitar.
Now 5 is an interesting number. When I first started playing bass, it really bothered me that they (standard 4 string electric basses) didn’t go as low as "the sound in my head". However, the more I played bass, the more I started hearing them for what they are, range-wise. Accordingly, the desire for a 5 has definitely dimmed over the years. The parts I hear in my head (without a bass in my hands) now translate right onto the 4 string and all is once more well in my world.
Even back when I was jonesin’ for a 5er a bit (occasionally quite a bit), I never did get one because I don’t like the look, people talked about needing a certain amp to do the low B justice and I was already tapped out for $ with 2 basses and 1 nice amp, none of the bands I like use 5 strings, the fretboards seem intimidatingly wide, I definitely don’t need any more weight in an instrument and did I mention I don’t like the look? However, the prospect of going much lower than guitar, really being able to take it down to another depth, does have its attraction.
Beyond that, also playing guitar makes me want a 6 string bass less, not more. However, if I had never played guitar, being the melodic kind of guy I am, I could see me wanting to take excursions up into the higher notes such as only a 6 string (or differently tuned bass with fewer strings) would allow if bass were my only means of musical expression. However, that would only be in a kind of solo home way — doing it in a band would be getting pretty far away from the type of music I actually enjoy listening to on a regular basis.
Overall, playing guitar makes me think guitars have 6 strings and basses have 4, and working from there, each is awesome in its own way