It is more likely that a 4mm ground glass nodule is benign, not serious if caused by a benign disease, and more serious if caused by a malignant tumor. Lung nodules can be categorized into solid nodules and ground glass nodules according to their density. Ground-glass nodules include pure ground-glass nodules and mixed nodules, which have both ground-glass and solid components. Solid nodules generally have a lower risk of malignancy, pure ground glass nodules have a higher risk of malignancy, and mixed nodules have the highest risk of malignancy. Nodule size is also associated with the probability of malignancy, with most 4-mm nodules generally being benign and only a smaller risk of malignancy resulting. If the 4mm ground-glass nodule is caused by pneumonia, tuberculosis, granuloma, malignant tumor, etc., it is not serious and can be improved or cured by timely treatment. If it is malignant bronchopulmonary cancer or lung metastasis, it is serious and requires active treatment. If you find 4mm ground-glass nodules, we suggest that you seek medical advice from a professional doctor.