Fever in four year olds is usually caused by viral, bacterial, or mycoplasma infections. Also some diseases of the immune system and blood disorders can cause fever. Patients need to check blood analysis, c-reactive protein, and mycoplasma pulmonale. First rule out blood disorders and immune system disorders. If there are abnormalities in the urine and stool, the urine and stool need to be checked to rule out urinary and intestinal infections. Patients are advised to go to a regular children’s hospital for examination and clear judgment. Then, according to the examination results, treat the symptoms. Babies with fever must pay attention to drink more water to avoid dehydration and eat more fruits and vegetables.