Please get a dental check up. A filling may be required or a tooth might be erupting. If cavity is very deep, your dentist will advise you . In children a lot depends on how cooperative the child is and we need to modify treatment accordingly.

Your child is just four years, only in six years permanent teeth starts erupting, so kindly consult with the Dentist immediately, see whether the toothache is due to decay or any infection, confirm the diagnosis and start the treatment... In this age can't do extraction, since permanent tooth grows in six years , so your child can't be without a tooth till six years... There is pulp therapy which is called as Pedo RCT which can be done, but have to confirm it with x-ray....