Yes, you can give Lax-A-Day™ to help with constipation in your 4-month-old baby. It is recommended to administer 1g per kilogram of body weight, up to a maximum of 17g per day. You can mix the powder in water without changing the taste or texture. It is best to give it in a small amount of milk at the beginning of a feeding and then give the rest of the milk to ensure the correct dosage is taken.