Is it obligatory to make up the prayers that were missed due to ignorance of how to perform ghusl from janabah for four or five years, and what is the expiation for that?

The issue of making up prayers that were performed despite the presence of something invalidating them is a matter of scholarly disagreement. The majority view is that they must be made up. However, Shaykh al-Islam Ibn Taymiyyah held that one who was ignorant of a condition or pillar is not obligated to make them up. The view of the majority is more cautious. Nevertheless, it is permissible to follow Shaykh al-Islam's opinion, and in that case, it is recommended to increase in supererogatory prayers and seek forgiveness. If you wish to be more cautious, then make up the prayers to the best of your ability, in order, to avoid the disagreement. It is permissible to make them up during times when prayer is ordinarily prohibited. As for ghusl on Friday, it removes impurity according to the Hanbalis, even though the preponderant view is that it does not suffice for obligatory ghusl. However, it is permissible to adopt the Hanbali view for ease.