**Core Concept**
In pediatric orthopedics, the treatment of choice for a fracture shaft femur in a child depends on the age of the child and the severity of the fracture. Children's bones have a unique ability to heal quickly and without the formation of significant callus, a process known as "physeal closure."

**Why the Correct Answer is Right**
In a 4-year-old child, the treatment of choice for a fracture shaft femur is closed reduction and casting (nailing is not suitable in children due to the risk of growth arrest). This method allows for the preservation of the growth plate and minimizes the risk of complications. The child's bone has a high capacity for healing, and the closed reduction and casting provide the necessary support for the bone to heal in the correct position.

**Why Each Wrong Option is Incorrect**
**Option A:** Intramedullary nailing is not the treatment of choice in children due to the risk of growth arrest and the difficulty in navigating the smaller bone diameter.
**Option B:** Open reduction and internal fixation is not typically required for a fracture shaft femur in a 4-year-old child, as the bone has a high capacity for healing.
**Option D:** External fixation is not typically used in the treatment of a fracture shaft femur in a child, as it can cause more damage to the surrounding soft tissues.

**Clinical Pearl / High-Yield Fact**
In pediatric orthopedics, it's essential to remember that the treatment of choice for a fracture shaft femur in a child depends on the age of the child and the severity of the fracture. In children under the age of 6, closed reduction and casting are often sufficient, while in older children, more invasive methods like intramedullary nailing may be required.

**Correct Answer:** C.