Can you tell us more about your diagnosis--why have you been given the prescription to use estrogen (I assume your doctor prescribed this, after a medical exam?).
What type of exam and/or testing was conducted, and what was ruled out? What information has your doctor provide regarding your amenorrhea for 4 years (did you breastfeed, and if so, how long?). Do you have any medical family history of amenorrhea or reproductive conditions?