Thanks for posting your query on health care magic.
Since the cyst
is only 4cm in size less likely it is cancerous.
Usually cyst more than 6cm, complexity on ultrasound
, increasing size with ascitis suggests malignancy.
I suggest you to get a RMI(risk of malignancy)test done. It actually predicts the likely chances of the cyst being malignant or cancerous
Hope this helps.