my bb 4 months old drop hair. is it normal ?

Yes, it is normal. This type of hair fall that you have observed is likely that of lanugo hair. Lanugo hair is soft thin hair that fetuses grow while they are in the mother’s womb. Lanugo generally vanishes between 36 and 40 weeks of gestations. If this hair is still on the baby after birth, they will usually be rubbed off naturally after two to four weeks or by the fourth month. After this, your baby’s hair will enter a new growth cycle. The hair that grows in this stage will be what your baby’s actual hair will be (and may likely appear different from the type he/she had when he/she was first born). The growth rate differs for every baby. For some, it may take up to two to three years to have a great head of hair.

Yes it's normal. Mine too. And at the same time this is also the time when mummies might be losing their hair too. I did. And I cut it shorter and it was better.