How can I treat coughing and colds in my 4-month-old?

Very carefully! Cough, cold and nasal congestion are very common in kids under 6 months. Recommended, safe treatments include nasal saline drops, vaporizer, elevating the head of the bed, and occasional nasal suction. If your baby has a fever, refuses to eat, is extremely irritable, or is wheezing, you need to see the pediatrician. Do not use over-the-counter cough and cold meds, even those sold for "infants".
TLC. Watchful waiting. Saline drops are helpful and safe. Avoid over the counter medications, unless your doctor approves. It may be worth visiting the doctor to make sure s/he is not wheezing or having any breathing issues.