It depends upon the age and your meenopausal status. If you are premenopausal and having regular menstrual cycles,then there is nothing to worry. It's likely to be functional cyst. You can follow up with sonograpy two months later.
If yu are post menopausal, then also the chances of it being malignant are very less. Please contact your gynecologist.
First off, to put your mind at ease, the CA 125 number can be a bit high due to other reasons. It's a fine thing to follow when a person is in treatment for ovarian cancer and the doctors can see if treatment is working as the number will decrease.
Having a cyst is not that uncommon and over 90% of ovarian cysts are not cancer. They're painful at times but we seem to get them all too often. I'm glad you've had it checked out. At 4.5 cm the doctor will probably monitor it. If it continues to expand and get over 5 cm, the doctor may talk about removing it. I had a 10 cm cyst plus my ovary and tube removed a few years ago. I didn't need it any more since I'm post menopausal but I was worried for awhile.
Keep in touch with your doctor. Let us know how it all works out.