My son’s hair is falling a lot. He is 4 months old now. Is this normal?

All those newborn hairs reach the end of their growth cycle at the same time. So many new hairs are growing from underneath, and pushing the newborn hair out. After about first year of life, the hairs on our heads are all at different stages in the growth cycle, growing at different rates, so they don't all fall out at once. Hormones tell our hair when and how to grow, but newborns experience a hormone drop after they are born.