I have been using Rotosound Swingbass strings on my 5 string and I am pretty happy with the E through G strings but the B is kind of "muddy." I usually use .45 to .130. Do you think going to a .125 would help?
Quite often it can be more down to the bass itself than the strings - the 4-string BG was a happy accident in many ways and just adding another string is a lot harder than it sounds. So - thinks like the pickup magnets and the stiffness of the neck can affect the B string, but not the other 4 strings.
I suggest using a taper wound B string. They are designed to sit snug in the saddle and sound tighter. Using a lighter guage string will work too, but the taper wound strings are better, IMHO. Treena