Most of the 4mm ground-glass nodules in the upper lobe of the left lung are caused by bacterial infections in the lungs, tuberculosis and tumor diseases. 1. Bacterial infection of the lungs: 4mm ground-glass nodules in the upper lobe of the left lung caused by bacterial infection are often accompanied by fever, cough, sputum, chest pain and other symptoms. 2. Tuberculosis: Tuberculosis is caused by Mycobacterium tuberculosis infection, which often results in the appearance of nodular shadows in the lungs, as well as cavities, corn-like shadows, and calcified spots in patients. It is often accompanied by low-grade fever, night sweating (abnormal sweating after going to sleep and stop sweating after waking up), fatigue, cough, hemoptysis and other symptoms. 3. Tumor disease: Lung tumors can also cause ground-glass nodules. Nodules <5mm are usually benign nodules, most of them have peripheral membrane, and they are spherical nodules or masses with smooth and sharp edges, which grow slowly or do not grow, and some of them may be absorbed on their own. The patient only needs to have regular checkups to see if there is any change in the size and shape of the nodule. If the patient has symptoms such as fever, cough, sputum, etc., it is recommended that the patient go to the hospital to clarify the cause of the disease and under the guidance of the doctor for treatment. If the patient has no obvious discomfort, it is recommended that regular reviews be conducted to observe the growth trend of the nodules to avoid progression of the disease.