She never hangs, because she confesses to witchcraft, blaming John Proctor, saying that "He wake me every night, his eyes were like coals, and his fingers claw my neck, and I sign, I sign". After this all of the attention turns onto John, and convicting him.
After this, there is no mention of Mary Warren for the rest of the play (Act 4). For a diary entry for her for Act 4, you could maybe describe her chagrin at Abby and Mercy skipping town, her regret for caving and blaming John, her sadness at his hanging and all of the people who suffered. Any other thoughts that you might have if you were Mary could be inserted-it could be refreshing to add some of that closure! Good luck!