For the common app, should the student label the essay "Prompt #4 "(or whichever) or just paste in the essay assuming the ad com can figure out which prompt is being addressed?
There’s no need to alert the AdCom to the prompt you are using. It should be evident in your essay. Also, it would use up precious words in your overall word count!
You might ask what happens if you choose Prompt #7, the open-ended prompt. Still, you have no need to indicate your choice. AdComs aren’t overly concerned with the prompt you are using. Rather, they seek an essay that will engage them from the first sentence and paint a vivid portrait of you the applicant.
Hope this helps!