EthCos

That's good advice thank you. The application instructions state that "three letters are required", but nothing about a maximum. I suppose it can't hurt to inquire about a fourth.

One of my rec letters was positive, but kind of short. I expected this to be one of my strongest letters. Due to this I may add an extra letter (beyond the three accepted on the app system). This professor wrote an amazing internship rec letter, so I know this extra letter will be great. I will have to contact admissions to arrange to have the letter sent rather than submitted electronically. Should I mention the reason for the extra letter? I did everything right with this professor (good relationship, helped with research beyond class, would frequently discuss the field with him, etc.) and the brief letter was a total shock. Also, do you think the trouble of adding a strong letter is worth it or will admissions committees be put off by the request?