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.