Must I make up the days I fasted in Ramadan four years ago, based on my mistaken belief that it was Istihadah, while it now seems more likely to me that it was menstruation?

The blood that precedes the menstrual period by a few days is considered menstrual blood, and your fasting during these days is not valid. You must make up for these days because they are a debt upon you. You are not required to pay an expiation (fidyah) if you delayed making up the fast until after the following Ramadan due to your ignorance of the impermissibility of the delay.