The size is a very important part of the diagnosis... but given your age the odds of any ovarian cyst being 'cancer' is very low. Simple septations (septums) in these cyst can also be completely normal and not cancer. If the cyst is large enough, it should be followed with another ultrasound in a few months. In addition, if there is high suspicion on your doctor that this maybe a worrisome cyst, a CA-125 blood test CAN be potentially useful, although again not diagnostic (this is because there are several things can can raise the Ca-125 level outside of ovarian cancer.). If the cyst is a reasonable size, close monitoring is appropriate. If the cyst is concerning to you main doctor, a second opinion from a GYN-Oncologist who can review the images and CA-125 level is another option to determine the next course of action.

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That is a good size cyst. Some ob/gyns would simply get the CA-125 and other tumor markers then proceed - if markers were negative then a laparoscopic removal of the ovary would be likely. Others would refer to a GYN-Oncologist again for a second opinion. The majority of these turnout to be cystadenomas (benign tumors that can get quite large); however because it is over 4 cm, it definitely needs to be followed closely. If the only other feature is the simple septations, that leads me to believe it is MORE likely to be a benign growth.. but unfortunately, even after doing all the tests and ultrasounds, there is just no way to know 100% sure if it is benign.. so if it persists, most doctors would recommend removal to find out for sure what it is and proceed from there.