What is the expiation for a vow to fast every Monday for someone who has been unable to fulfill it due to illness for four years?

Fulfilling a vow to fast every Monday is obligatory if it was conditioned upon an event that has occurred. If the inability to fast is permanent, then an expiation for breaking an oath (kaffarat yamin) is required (feeding ten poor persons, clothing them, or emancipating a slave; or fasting three days if one cannot afford any of these). Some scholars have added feeding one poor person for each day. However, if the inability is temporary, then the days that were not fasted must be made up once the inability ceases.