I know a really super bass player, who has played on 4 strings all his life. He owns about a dozen of them. He also owns a 6-string, but finds he never uses the extra strings. Why not? Well, because when he learned bass, those strings weren't there. He hasn't learned much for a while. Why should he? He's good at what he does, which is all he plays anyway.
Kind of like a washtub bass player explaining that "he's never needed to play more than one note at a time in the past, so why have more than one string?" But of course he "never needed" those extra strings - he never had them.
Imagine, from the inverse perspective, if he'd developed a style that makes heavy use of 2-octave arpeggios, and then *lost* a couple of strings. Ever wanted four hands? Imagine growing up with them, then losing two. You'd be crippled.
Four strings make some styles of play (like slapping) easier, while six strings are best for other styles. If I had to choose one, I'd choose six strings, because this means I ALSO have a 4 and a 5, already built in.