I don't think the size can tell you if it is cancerous. I know that the doctor can get a good idea from the ultrasound (but not always) whether or not a cyst is benign, but I don't think size is what assists in their decision. They look to see if the cyst is fluid filled or solid.
Sorry, I don't know if a 4 cm cyst is large. I think my cysts are only 2 cm, but they don't cause me any pain or anything.
4 cm is slightly under 2 inches. It's not considered "large," but that doesn't necessarily mean a thing in terms of your medical condition.
Size is not really an indicator of malignancy, far as I know. The suckers can get to the size of bowling balls or worse. So can fibroids. If you go to Google, set it to "images" and the input "uterine cyst" and/or "fibroid", you will be blown away by some of the photos!
However, let me emphasize that small size does not automatically mean harmless or meaningless. Small sized cysts and fibroids and other kinds of growths can cause beastly symptoms, and there is always the question of malignancy.