What is the ruling regarding the four years in which I did not fast the days of menstruation during Ramadan, and must I fast them with or without expiation?

A menstruating woman must make up for the fasts she missed in Ramadan, and expiation (kaffarah) does not suffice if she is able to make them up. You must make up all the days you owe from the past four years. If you cannot ascertain the exact number, you should err on the side of caution until you are reasonably certain that your obligation has been fulfilled. You are not required to pay expiation for delaying the make-up fasts, as the delay was due to ignorance.