Plan year means the year that is designated as the plan year in the plan document of a group health plan, except that if the plan document does not designate a plan year or if there is no plan document, the plan year is -
(1) The deductible or limit year used under the plan;
(2) If the plan does not impose deductibles or limits on a yearly basis, then the plan year is the policy year;
(3) If the plan does not impose deductibles or limits on a yearly basis, and either the plan is not insured or the insurance policy is not renewed on an annual basis, then the plan year is the employer's taxable year; or
(4) In any other case, the plan year is the calendar year.