I play in a relatively progressive metal band using a four string on songs that have seven string guitars. I tend to find that when I do mess around with a five, I fall into the trap of just playing the obvious roots because they're available. I see it with a lot of other metal players too. Playing four makes me move up the fretboard, and actually write something I might have otherwise not. Of course it's all personal preference, but you don't "need" those notes. They're available elsewhere in a different octave that could serve the song just as well, although differently.
Strictly speaking of string count, a five makes a four obsolete. As whole instruments, they're just a little bit different, and a five can't always perfectly replace certain fours, especially in feel or in the case where the same bass isn't available in both.
Strictly speaking of string count, a five makes a four obsolete. As whole instruments, they're just a little bit different, and a five can't always perfectly replace certain fours, especially in feel or in the case where the same bass isn't available in both.