That was my thinking. I just had gigs here and there where the 6 wasn't working, but there's not much you can't transpose an octave in a pinch.When I gig, one of the basses is a five and the other a four. I learned on the five, so I can sometimes be challenged by the four on some songs. It happens less now because I do most of my practicing on the four, on purpose. There are very few songs that pretty much require a fiver. I suppose if I was trying to play Summer of 69 on my four I could always just use the normal D instead of the D on the B string. Most people wouldn't notice it. I don't do "drop D", for obvious reasons.