She is probably having a big shed. Some Bs have an annual or bi-annual shed where bunches of hair come out, and sometimes there is fluffy undercoat that looks kind of white.
Another possibility is that her thyroid is failing. Three or four years old is the typical onset of thyroid problems. Thinning hair, and dry skin are common symptoms, as is weight gain or loss.
Is her hair coat actually changing color? Is it really white? Or just a washed out tan? If it is really coming in white, like pointer or dalmation white, her problem could be something much more rare. I know of someone who had this situation with a B….if that is the case, I can put you in touch with her.
Hopefully, it is just a seasonal hair coat change. Sometimes something stressful with trigger this...maybe the baby arriving?
Good luck, let us know how it goes....