The following therapeutic measures can be taken when the four-year-old baby teeth become black: 1.Enhance the enamel hardness: When the surface of the milk front teeth forms extensive caries, you can apply smooth pulp needle to adjust the edge, apply sodium fluoride, stannous fluoride and other solutions to enhance the enamel hardness to avoid the expansion of caries. 2.Restore the original color: When the child’s physical quality is weak and often takes Chinese medicine preparations to cause the milk teeth to stain, you can apply polishing abrasive head, dip polishing paste and rub to remove 3.Repair the defect: If the baby gnaws on hard objects to form a tooth with hidden crack and cause pulp necrosis, you can grind open the pulp cavity, remove the infected pulp, use calcium hydroxide paste to fill the root canal and repair the defect. 4.Wear a gap retainer: If the baby tooth forms a residual root and crown with repeated infection, you can pull it out and wear a gap retainer to wait for the replacement of permanent teeth.