For a four-month-old baby, the dosage of this medication (colistin) must be calculated precisely by your paediatrician based on your baby's exact weight, not just their age. Giving an incorrect dose can be unsafe or ineffective. Please contact your prescribing doctor immediately to get the correct dosage and schedule. In the meantime, focus on keeping your baby well-hydrated by continuing frequent breast or formula feeding. You can also ask your doctor about using an Oral Rehydration Solution (ORS). Seek immediate medical attention if your baby shows signs of dehydration, like extreme sleepiness or fewer wet diapers.