Is taking 4mg of folic acid and biotin good for my hair?

No: One's hair will be "as good as it gets", if one eats a normal, healthy, balanced diet (which will contain all the folate (folic acid) and biotin needed for normal living). If a person has a rare disorder causing a deficiency of folate (folic acid) or of biotin, she can work out the details with her doctor. Normal people do not need extra doses of supplements, but 1 daily multivitamin or 1 prenatal vitamin is okay to take.