Drop B is a half step below Drop C, so the strings feel even looser. The right gauge depends on your target feel, scale length, construction and setup. Some players tune a 4-string to B E A D instead, depending on the range they want. The lower you go, the more gauge, fit and setup matter.
For a full set, Power Wound Nickel is a versatile nickel-plated roundwound, and Power Steel Stainless adds brightness, attack and note definition. S.I.T. Single Strings let you go heavier on just the lowest string.
Drop B isn't the same as a 5-string bass. Both reach a low B, but a 5-string adds the B below a standard E A D G, while Drop B lowers every string and changes the interval between the lowest two strings.
If a bass will stay in Drop B, choosing its gauges and setup around that tuning gives a more consistent feel. Evaluate how it plays and have a technician check it if you're unsure.