It is fine to correct your child's grammar as soon as he or she is putting together a sentence of even three words. You just have to know how to do it. The method is called remodeling. You can ask it as a question and wait for a nod from your child or you can just repeat the sentence directly after your child has spoken. In this way your child hears the correct grammar and pronunciation without actually being told what they are saying is wrong.
Children are experts at language acquisition and can readily make the changes.
If your child is four, and is still makes many grammatical mistakes similar to the example I gave, then there may be a language problem. If you are concerned, it is certainly worth having him or her evaluated by a language specialist. Generally, this is a problem that is easily fixed by language enrichment when it is caught at an early age. You should not rely on this information as a substitute for personal medical attention, diagnosis or hands-on treatment. If you are concerned about your health or that of a child's, please consult your family's health care provider immediately and do not wait for a response from our professionals.