For a 4-month-old baby who is constipated, you may consider giving them PEG-3350, which is available under various brand names such as Lax-A-Day and Restoralax. This is a tasteless and odorless powder that can be mixed into any liquid, including breast milk or formula, based on the baby's weight. Additionally, it is important to encourage the baby to drink fluids regularly and try gentle exercises like bringing their knees to their chest or mimicking a bicycle pedaling motion while lying on their tummy. If the constipation persists or if you have any other concerns, it is recommended to consult a healthcare professional for further guidance.