| The nut could be weak or hollowed (The plastic thing just before the headstock.), or the action on your "4th" string could be very low. Or, of course, it could be detuned way too much. Get a digital tuner and try tuning it to G. If it still rattles, take a look at the nut (plastic thing). If the part that your string has to through appears damaged then take a photo or take it into a music shop. And if neither of those is it, look at your bass from "profile" view (side) and see how high that string is from the frets. If it looks to be lower then the rest, that's the cause.
It could be more things, I'm just offering some advice from what I know. Good luck. |