PSEUDOMYXOMA PERITONEI is a condition of mucinous ascites. This occurs usually from the rupture of a tumor of the appendix. it is an easy diagnosis to make at surgery. Also a CT scan would show fluid and low density material around the abdomen.
wikipedia has a nice summary: http://en.wikipedia.org/wiki/Pseudomyxoma_peritonei
If your ultrasound or Ct scans have not shown this, it is unlikely that you have this condition.
a borderline tumor per se would affect your fertility.
For a small cyst such as a 4 cm cyst, waiting until September is not unreasonable. Also it would be good to repeat the ultrasound to make sure your cyst is not just a cyst of ovulation that resolves on its own