It's used to be able to play a low D on a 4 string. You can get a Hipshot Xtender Key that replaces your tuning peg, you just flip a lever and you have low D, flip it back and you have E again. I used to put them on all my 4 string basses, but I play 5 strings these days. I still have one of the Hipshots here, it's a black Gotoh style tuner if you're interested PM me.
Chris A.
