hello user i understand your concerns . she might be harboring a recurrent depressive disorder , as suggested by your description - in this disorder these kind of episodes keep on occurring and this is a common trend that future episodes became of longer duration , more severe symptoms and difficult to treat - so to tackle this disorder the treatment not only should be taken during acute episode only but need to be continued further ( as she had 4th episode, if she would be my patient - i would like to put her on prophylaxis with a mood stabliser drug like lamotrigene ) also may be there are some psychosocial issues , so behavioural therapies can also be implied from your psychiatrist / psychologist - some physical disorders like thyroid disorders can also be contributory and evaluation for underlying bipolarity should also be done .....in any case if depression does not responds to antidepressants - she may be put on on combination of antidepressants or with mood stabliser and antipsychotic drugs or electroconvulsive therapy- so discuss the issues with your psychiatrist - take care