My 4year old has a grey front tooth. should i be worried as it's a milk tooth? what can be the causes?

Gray tooth : The most common cause for a grey primary or "milk" tooth is dental trauma. At some time in the past your child accidentally banged this tooth against a hard object. The grey color means that there has been disruption to the tooth's blood supply causing pulp damage. As long as the tooth doesn't hurt or become abscessed, no treatment may be needed. Have them examined by a pediatric dentist.