Hi Sapna, As your son s age 4 year, I don t think he need RCT for his cavity. Because all of his teeth are Primary teeth (also called baby teeth or deciduous teeth). Primary teeth are smaller in size and whiter in color than the permanent teeth that will follow. By the time a child is 2 to 3 years of age, all primary teeth should have erupted. 20 in number.(10 in the upper jaw and 10 in the lower jaw). Shortly after age 4, the jaw and facial bones of the child begin to grow, creating spaces between the primary teeth. This is a perfectly natural growth process that provides the necessary space for the larger permanent teeth to emerge. Between the ages of 6 and 12, a mixture of both primary teeth and permanent teeth reside in the mouth. So all his teeth are fall/shed. and new permanent are appear. First of all gargle/vigorously swishing with the worm salty water around his mouth frequently.The salt should only be about 1-2 tsps per 6oz of warm water and it can only continue to heal if it is not washed off. If you believe in Homeopathy then Along with it give him Marc sol 200 /3 dose daily , prepare in sugar of milk ,for 3 consecutive days. Brushing at least twice a day, and flossing at least daily.Avoid constant sipping of sugary drinks or frequent sucking on candy and mints. You also remember that decay and infection in baby teeth can cause damage to the permanent teeth developing beneath them. So if nothing help Tooth extraction can also be the treatment for extreme decay. Hope it help. Dr. Rajesh Sarkar rs787878@gmail.com