It is unfortunate, although not surprising to me, that your information appears limited to one and two year Leaves of Absence.
While I do not suggest that you are responsible in any way for research, record keeping and/or disposition, your apparent inability to produce such a record(s) tends to support my tentative conclusion that such, to me troubling, lengthy FOUR YEAR Leave(s) of Absence may be without, at the very least, recent precedent.
Perhaps if the Supervisor puts an additional "Research Stipend" on the Monday's Agenda, with the incentive of compensation, he can research his long career's records and provide the readers with an answer to the above.