Originally Posted by Slowgypsy
Using the bottom 4 strings of a 5-string set is a common and very workable idea. However... if you try to use a gauge string that's larger than the corresponding nut slot... especially the outer two strings... that's a recipe for a cracked nut, not to mention much higher action.
Very true. I mentioned filing the nut to accommodate, but doing so is permanent, hence thinking about it. I should have mentioned the down side to putting those strings on without filing the nut. My bad.
To the OP, if you don't want very floppy strings, you'll need a much thicker gauge than typically comes on a four string bass. If you want to use, say, .110 or even .115 gauge E strings tuned to B, they're still going to be quite loose. You will need to raise your action significantly and adjust your truss rod a good bit.
You could always get a 5 string bass and solve this altogether as well