Four-year-old children’s teeth are rotten, may be related to poor oral hygiene, may also be due to dietary factors, dental development abnormalities and other reasons. 1. Poor oral hygiene: four-year-old children usually do not pay attention to oral hygiene, do not develop a good habit of brushing their teeth every day, resulting in a large number of food debris accumulated on the surface of the teeth, or in the interstices of the teeth in the stagnation, over time, may cause dental caries, which leads to tooth decay. 2. Dietary factors: If a four-year-old child consumes a large amount of sticky, sugary soft food for a long time, it is easy to stick to the teeth, and acidic metabolites are likely to be formed, resulting in demineralization of tooth enamel, which can cause teeth to rot. 3. Abnormal dental development: common in enamel underdevelopment, mainly due to trauma, infection, severe nutritional disorders and other factors, due to enamel underdevelopment, resulting in dentin and enamel is relatively weak, and the degree of calcification is low, it is easy to cause tooth rot. Four-year-old child teeth are rotten, may also be caused by other reasons, it is recommended to seek medical advice as soon as possible, by the doctor’s guidance and active treatment.