Nodules can occur in the lungs for a variety of reasons. Most are
infectiousInfectious endocarditis
Infectious mononucleosis
Infectious mononucleosis #3 and not cancerous. A 4 mm nodule is tiny. Unless the previous CT scan was done with the same machine and in the same fashion, the nodule might not have been visualized even though it was present. The rate of growth varies considerably, depending on the type of lesion. A nodule that is less than 10 mm in diameter and unchanged over nearly 18 months would almost certainly not be cancerous.
The best advice I can give is that you be seen jointly by a pulmonologist and a radiologist who specializes in the chest. You will get a much better picture of the likelihood of the nodule being non-cancerous or cancerous. They will also give you advice on what to do. At this point, waiting 3 months for follow-up would be a reasonable.