Seriously, if you're playing a 4-string all the way down in drop B, you might as well just buy a 5-string bass. Drop D on a 4-string is fine, but if you're going down that far, just get a 5-string. You'll thank yourself later for not having to detune and retune and do setups all the time.
If that isn't an option, at least get a 5-string set of strings so you have a real B string instead of having to drop tune a string too small for the task. You can do that easily without custom orders; Rotosound and D'Addario make standard 5-string sets that are both relatively cheap and have good tension. You can also order single B strings from Link Removed but that's not as economical as buying full sets, especially if you're using full drop BEAD tuning on your 4-string.
Just my thoughts, anyway.