Is it ok using baby oil after 4 months of hair transplant ?

You don't need to use baby oil at 4 months mark after a hair transplant.  However, it is harmless and at this point you can treat your scalp as you used to before your hair transplant.

It is safe to apply baby oil to your scalp four months after your hair transplant.  I would only caution that the scalp has abundant sebaceous oil glands already and does not usually need extra oil except in rare cases of eczema and psoriasis.  In fact, dandruff/flaky scalp is believed by dermatologists to be partly caused by inflammation resulting from excess oil and yeast overgrowth.  Lack of adequate cleansing or washing with shampoo can result in excess scalp oil and consequent dandruff.  It is best to control scalp flaking with a dandruff shampoo which will control the excess oil and yeast.