Viral Upper Respiratory Tract Infection (URTI): This is the most common cause of cough and low-grade fever in children, often due to viruses such as rhinovirus, coronavirus, or adenovirus. The symptoms are usually self-limiting and include runny nose, cough, and mild fever.

Acute Bronchiolitis: Common in young children, especially under the age of 2, but can occur in 4-year-olds, particularly if they have underlying respiratory conditions. It's usually caused by the respiratory syncytial virus (RSV).
Pneumonia: Can be caused by bacteria (like Streptococcus pneumoniae) or viruses. Symptoms include cough, fever, and difficulty breathing. The severity can vary from mild to severe.
Asthma: If the child has a history of wheezing or asthma, an exacerbation could present with cough and low-grade fever, especially if triggered by a viral infection.
Allergic Rhinitis: Can cause chronic cough due to postnasal drip, and if there's a concurrent viral infection, a low-grade fever might be present.

Pertussis (Whooping Cough): Although less common due to vaccination, it's crucial to consider, especially if the vaccination status is unclear or if the child is not up to date with vaccinations. The cough can be severe and is characterized by a whooping sound.
Tuberculosis (TB): While rare in many areas, TB is a critical diagnosis not to miss, especially if the child has been exposed to someone with TB or has recently traveled to an area with high TB prevalence.
Foreign Body Aspiration: Can cause sudden onset of cough and respiratory distress. It's an emergency that requires immediate attention.
Pneumocystis jirovecii Pneumonia (PCP): In immunocompromised children, PCP can present with cough, fever, and respiratory symptoms.

Cystic Fibrosis: A genetic disorder that can cause recurrent respiratory infections, chronic cough, and failure to thrive. It's rare but important to consider in children with persistent or recurrent respiratory symptoms.
Immunodeficiency: Conditions like severe combined immunodeficiency (SCID) or HIV can predispose children to recurrent, severe, or unusual infections.
Congenital Heart Disease: Certain conditions can lead to increased pulmonary blood flow or congestion, presenting with cough and respiratory symptoms.
Interstitial Lung Disease: A broad category of rare diseases affecting the lung tissue, which can cause chronic cough, difficulty breathing, and other respiratory symptoms.