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

yes it is normal because your baby is always lying down and the head keeps getting into a lot of friction. hence it leads to hair fall. i would suggest that you comb your baby's hair everyday at least twice, and try to switch position through the day. also, take your baby out in the sun and massage the head regularly, as it improves blood circulation and may reduce hair fall and promote hair growth.