(4)
The Executive Director shall prescribe the time, form, and manner in which an employee may specify—
(A)
the total amount such employee wishes to contribute under this subsection with respect to any particular period referred to in paragraph (2)(B); and
(B)
the period of time over which the employee wishes to make contributions under this subsection.
The employing agency may place a maximum limit on the period of time referred to in subparagraph (B), which cannot be shorter than two times the period referred to in paragraph (2)(B) and not longer than four times such period.