Sec. 4. Self-insurance plans for state employees involving income disability insurance, principal amount accident insurance, or both, must not, as to the form or forms of the insurance, discriminate between the employees of any department, commission, board, division, facility, institution, authority, or other establishment, except that the contributions for the insurance and benefits from the insurance may be equitably graduated in relation to:
(1) the employment compensation schedule; and
(2) if actuarially justified, the employee's age.