Viral Gastroenteritis: This is the most common cause of vomiting, headache, and body aches in a 4-year-old, often due to norovirus or rotavirus, characterized by acute onset of symptoms, fever, and sometimes diarrhea.

Influenza: A common cause of headache, body aches, and sometimes vomiting in children, especially during flu season, accompanied by fever, cough, and sore throat.
Strep Pharyngitis: Although more commonly associated with sore throat, strep pharyngitis can cause headache and body aches, and occasionally vomiting, especially if the child has a high fever.
Urinary Tract Infection (UTI): Can cause abdominal pain, vomiting, and sometimes headache, especially in young children who may not be able to verbalize their symptoms accurately.

Meningitis: A life-threatening condition that requires immediate medical attention, characterized by severe headache, stiff neck, fever, and vomiting. Although less common, it is critical not to miss this diagnosis.
Appendicitis: Abdominal pain, vomiting, and sometimes headache can be symptoms of appendicitis, especially if the pain localizes to the right lower quadrant.
Type 1 Diabetes: Can present with vomiting, abdominal pain, and sometimes headache due to diabetic ketoacidosis, a medical emergency.

Brain Tumor: Although rare, brain tumors can cause chronic headaches and vomiting, especially if there are associated neurological deficits.
Leukemia: Can present with non-specific symptoms such as headache, body aches, and vomiting due to bone marrow infiltration and metabolic disturbances.
Inborn Errors of Metabolism: Certain metabolic disorders can cause episodes of vomiting, headache, and body aches, often triggered by infections or dietary factors.