POI suddenly asked for a 4th LOR today

[Bactrian]

I spoke on the phone today (Dec. 5) with a POI at one of my top choices, a school ranked in the top 5 of my field. In the conversation he asked if it would be possible for me to submit a fourth letter of recommendation. I indicated that I would try my hardest to get one but the late date (application is due Dec. 15) may present difficulties.

Is this something I should seriously try to make happen? I'm afraid that between the late date and the fact that I'd be going to someone who I initially decided not to ask for a letter (different field, not 100% sure what she thinks of me though we had good interactions and I did well in her seminar) I'm worried I'll get a mediocre letter that will do more harm than good. I've spoken to a mentor (a senior tenured faculty member) who advised me to say polite things and then disregard the request. His argument was that I'll get an offer off of three letters or not at all, a fourth wouldn't magically make me more desirable. Does anyone have any thoughts?

In case it matters I'm in history and I just finished up an MA.

[kaputzing]

I agree with your mentor. If you don't have someone who'll make you out to be, I dunno, the next Hobsbawm in the making (not that I know what period you're interested in ), don't go for the fourth.