The severity of 4mm lung nodules varies from person to person. Benign lesions, such as old foci and localized non-specific inflammation, are usually not serious. However, if the diameter of the lung nodule increases gradually during the review, it is likely to be malignant, and the condition may be more serious and even life-threatening. 1. The condition is relatively not serious: the diameter of lung nodules of 4 mm may be due to a variety of reasons, such as old foci, benign tumors in the lungs and other related diseases, generally speaking, the condition is not serious. 2. Relatively serious condition: 4mm diameter lung nodule may be caused by early stage lung cancer, under normal circumstances, the lung nodule will gradually increase in the process of reviewing chest CT, if it belongs to the above situation, it usually indicates that the condition is relatively more serious, and may even be life-threatening. 4mm lung nodules, in any case, should receive regular follow-up, so as not to delay the condition.