Bad breath in a four-year-old child may be caused by poor oral hygiene, cavities, mouth ulcers and other reasons. 1. Poor oral hygiene: If children do not pay attention to oral hygiene, there may be a large number of bacteria in the mouth to multiply the phenomenon, which can lead to bad breath. 2. Dental caries: If there are cavities, food residues may remain, leading to bacterial proliferation and bad breath. 3. Mouth ulcers: If a child develops mouth ulcers combined with bacterial infection, bad breath may also occur. Parents are advised to take their children to regular hospitals in time to clarify the cause, and if treatment is needed, standardized treatment should be carried out under the guidance of the doctor, so as not to delay the condition of the child to bring adverse effects.