I have a 4cm ovarian cyst on my left side. How serious is it?
Likely, not very.
It is normal for the ovary to undergo monthly changes that allow for the development of cystic, collections of fluid, within them. For a young patient such as you, it would be best to be followed conservatively if this is not giving any symptom of pain, pressure, bladder or bowel changes and most importantly if you are not pregnant or are sexually active and not on any form of birth control.