A four year old child with a decayed tooth that has reached the nerve will usually experience severe pain, at which point a root canal is usually recommended to achieve pain relief and healing. A decayed tooth with nerve involvement is a very serious condition that usually results in severe pain. There are a number of reasons for this, with common causes including chronic lack of oral hygiene and attempts to consume large amounts of sweets. After the nerve has been damaged, a root canal is typically utilized. First, the necrotic pulpal nerve tissue is removed and the root canal is filled with a specialized medication, followed by a filling and repair. This provides quick pain relief and also repairs the tooth to avoid food impaction. In your daily life, you should pay attention to oral hygiene, develop a good habit of brushing your teeth in the morning and evening and rinsing your mouth after meals. At the same time, reduce the consumption of sweets and go to the hospital for regular dental checkups to prevent tooth decay from the root.