Musculoskeletal strain: This is the most common cause of back pain in children, often due to overuse, poor posture, or minor trauma. In a 4-year-old, it could be related to normal childhood activities.

Urinary Tract Infection (UTI): UTIs can cause back pain in children, especially if the infection involves the kidneys (pyelonephritis). Young girls are at higher risk due to their anatomy.
Constipation: Chronic constipation can lead to back pain due to the pressure exerted on the spine by a full bowel.
Scoliosis: Although often asymptomatic, significant scoliosis can cause back pain. Early detection is crucial for effective management.

Spinal cord tumors or abscesses: These conditions are rare but can be life-threatening or lead to significant morbidity if not promptly diagnosed and treated.
Osteomyelitis or discitis: Infections of the bone or disc can cause severe back pain and have serious consequences if not treated appropriately.
Child abuse: Unfortunately, child abuse can manifest as unexplained back pain among other symptoms. It's crucial to consider this in the differential diagnosis.

Scheuermann's kyphosis: A rare condition that affects the spine, causing an abnormal curvature of the upper back.
Ehlers-Danlos syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Back pain can be a symptom due to joint hypermobility and musculoskeletal issues.
Juvenile idiopathic arthritis: Although more commonly associated with joint pain and swelling, some forms can cause back pain due to inflammation of the spine.