Four-month-old babies may have reached the teething stage and are easily irritable, manifesting themselves by crying easily, not eating milk, biting and grinding their teething beds, drooling and so on.
Parents do not need to worry, just observe whether the baby has fever, or other discomfort, add supplementary food to the baby appropriately, or massage the teething bed to soothe the baby, to help the baby pass this period peacefully.
If your baby has a severe fever, or is in severe pain, then a visit to the paediatrician is recommended.