Final answer:
A patient with four major depressive episodes in the past five years should remain on antidepressant medication for at least 12 months, and possibly longer or indefinitely to prevent relapse.
Step-by-step explanation:
For a patient who has experienced four major depressive episodes within the past five years, it is generally recommended that they should remain on antidepressant medication for at least 12 months following remission of their most recent episode. In some cases, considering the chronic nature of their condition, treatment may be extended to several years or even indefinitely. This duration is advised to prevent relapses, and the decision to discontinue or maintain treatment should always be made in collaboration with a healthcare provider, taking into account the patient's unique medical history, response to medication, and potential side-effects.