An orthodontist, or a dentist with additional training evaluation, is the right person to answer this question case by case. A 4 years young child might need a space maintainer, which is in fact a form of braces. Most of the time orthodontic treatment might be recommended during the mixed dentition stage. That is when the child has lost a bunch of baby (deciduous) teeth, and a number of the permanent teeth have erupted into the mouth. This usually happen between the age of 8-12 for boys, and even earlier for girls.
The early treatment is called phase one. Most of the time this is accomplished with appliances (retainer like devices) or with dental braces. The first phase most of the time deals with enhancing jaw growth and/or attempting to intercept potential problems with the permanent teeth. After phase one is completed, in most cases, we observe the kid until all permanent teeth fully erupted to start the second phase. That varies depending on the pace of growth that is normally between the age 12 and 14.