Pneumonia: This is the most likely diagnosis given the combination of cough, fever, chest pain, and yellow sputum. Pneumonia is an infection of the lungs that can be caused by bacteria, viruses, or fungi, and these symptoms are classic presentations.

Acute Bronchitis: Often presents with cough and yellow sputum, and can be accompanied by fever and chest pain, especially if the bronchitis is severe or leads to a secondary infection.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: Patients with COPD can experience worsening of symptoms such as cough, sputum production, and shortness of breath, often triggered by infections.
Influenza: Can cause cough, fever, and chest pain, especially if there is an associated pneumonia or if the influenza virus directly affects the lungs.

Pulmonary Embolism: Although less common, pulmonary embolism can present with sudden onset of chest pain and cough, and it is crucial to consider this diagnosis due to its high mortality rate if untreated.
Tuberculosis (TB): Can present with chronic cough, fever, chest pain, and sputum production. TB is a critical diagnosis to consider, especially in high-risk populations or areas with high TB prevalence.
Lung Abscess: A localized, suppurative infection within the lung parenchyma, which can present with fever, cough, and chest pain, often with foul-smelling sputum.

Cystic Fibrosis: A genetic disorder that can lead to recurrent respiratory infections, chronic cough, and sputum production, but it's typically diagnosed in childhood.
Bronchiectasis: A condition characterized by damaged, dilated airways, leading to chronic cough, sputum production, and recurrent infections. It's less common and often associated with underlying conditions.
Sarcoidosis: An autoimmune disease that can affect the lungs, causing cough, chest pain, and fever, but it's less likely given the acute presentation and presence of yellow sputum.