Not serious. A ventricular septal defect of 4 mm is not serious, is a mild case, has normal pulmonary artery pressure, is usually asymptomatic and has a good prognosis. Ventricular septal defects are mainly caused by genetic abnormalities. According to the size of the defect and the shunt flow, it can be divided into 4 categories. 1. Light: small shunt volume, normal pulmonary artery pressure, asymptomatic and good prognosis. 2. Defect 0.5~1.0cm: some increase in right ventricular and pulmonary artery pressure, but symptoms are mild. 3.>1.5cm: high fractional flow, high pulmonary resistance, significantly higher pulmonary artery pressure, susceptible to infectious diseases, palpitations and other symptoms. 4. Huge defect with pulmonary hypertension: it will lead to Eisenmenger syndrome, and cyanosis, dyspnea and so on. The treatment of ventricular septal defects is mainly based on the size of the fractional flow and the size of the defect to judge and choose, can choose interventional therapy or surgical treatment such as ventricular septal defect repair. If discomfort occurs, immediate medical attention is recommended.