Praise be to Allah.
You made a mistake by not making up those days all this time. This (i.e. menstruation) is something which Allaah has decreed for the daughters of Adam and there is no shyness in matters of religion. You have to hasten to make up those four days, then in addition to making them up, you have to offer expiation, which is to feed one poor person for each day, by giving two saa’ of the usual staple of the country to one or more poor persons.