An employer may request medical certification for FMLA leave no more than once per year. While it is unfortunate that the employer misplaced the paperwork from four years ago, it remains the case that (a) they can ask for new medical certification and (b) by allowing four years to pass without requesting recertification, they've imposed a much lower burden on her than they had the right to impose under the law.
If the employer is requesting the new medical certification in a time frame of less than fifteen days, the employee would be within her rights to request additional time.

