If your 4-month-old baby had a fever and is now not sleeping well, it may be due to lingering discomfort from a mild infection, vaccination, or general irritability, and the safest approach is to keep the baby comfortable by continuing regular breastfeeding or feeds, ensuring good hydration, and maintaining a calm, cool sleeping environment. For fever or discomfort, only give a doctor-recommended dose of Paracetamol (acetaminophen) if needed, and avoid giving any other medicines without medical advice. Gentle measures like light clothing, lukewarm sponging, and soothing (holding or rocking) can help improve sleep. However, if the fever persists, returns, the baby feeds poorly, cries excessively, becomes unusually sleepy, or shows symptoms like vomiting or breathing difficulty, you should consult a paediatrician promptly, as young infants need careful evaluation.