The authority may provide that up to 25% of the dwelling units in any development providing low and moderate income housing under federal, state or local government programs be made available to low income families and persons whose probable aggregate yearly income is less than 4 times as large as the regular yearly housing charges for such dwelling units. The dwelling units shall be made available to these families and persons at monthly rental payments or carrying charges so that yearly housing charges do not exceed 25% of the probable aggregate yearly income of such persons or families. The authority is authorized to contract with sponsors or owners of housing developments for up to 3,750 such units during the fiscal year ending June 30, 1967.