Hi Pixie. I am sorry to hear that you are left with only part of one ovary and have been diagnosed with stage 4 endo. Was your surgeon able to excise all or most of the endo implants?
Hopefully, it will just be a matter of time before that partial ovary figures out that it has to get to work! However, as your surgeon said, you may need intervention conceiving. It is also possible that your partial ovary could stop functioning altogether throwing you into a surgical type menopause. Only time will tell. I don't recall your age (if you mentioned it before) but surgical menopause is associated with a number of increased health risks and these are magnified in younger women. Hormone "replacement" therapy has been shown to mitigate some of those risks.
Hopefully, your other ovary will figure it all out soon and you will be able to conceive!