Viral Upper Respiratory Tract Infection (URTI): This is the most common cause of sudden onset cough, sore throat, runny nose, and fever in a child of this age group. The symptoms presented are typical of a viral infection, which is often self-limiting and managed symptomatically.

Influenza: Given the sudden onset of symptoms, influenza should be considered, especially during flu season. The presentation of cough, sore throat, runny nose, and fever is consistent with influenza.
Allergic Rhinitis: If the symptoms are persistent and not clearly linked to an infectious cause, allergic rhinitis could be a consideration, especially if there's a history of allergies or if the symptoms worsen with exposure to specific allergens.
Pharyngitis: Caused by either viral or bacterial pathogens, pharyngitis could explain the sore throat and fever, with the cough and runny nose being secondary symptoms.

Epiglottitis: Although rare, epiglottitis is a medical emergency that can present with sore throat and fever. It's crucial to consider this diagnosis due to its potential for airway obstruction, even though the classic presentation includes severe dysphagia and drooling.
Pertussis (Whooping Cough): Especially if the cough is severe or has a characteristic whoop, pertussis should be considered, particularly if vaccination status is incomplete or unknown.
Pneumonia: While the primary symptoms suggest an upper respiratory infection, pneumonia can sometimes present with similar symptoms, especially if there's an associated viral or bacterial lower respiratory tract infection.

Tuberculosis (TB): Although rare in many parts of the world, TB can present with chronic cough, fever, and other systemic symptoms. It's essential to consider TB in the differential diagnosis, especially if there's a history of exposure or if the patient is from an area with high TB prevalence.
Lemierre's Syndrome: A rare condition characterized by a bacterial infection of the tonsils followed by septic thrombophlebitis of the internal jugular vein, which can present with sore throat, fever, and systemic symptoms.
Kawasaki Disease: An acute febrile illness of childhood that can present with fever, sore throat, and other systemic symptoms, including rash, conjunctivitis, and lymphadenopathy. It's a diagnosis to consider in the absence of a clear infectious cause and the presence of specific clinical criteria.