More than 3 letter writers?

[scarvesandcardigans]

Hi fellow linguophiles - 

I am interested applying to a PhD program in Romance Linguistics. I have already secured my 3 letter writers and the deadline is 12/8, so ~3 weeks from now. This morning I was asked by my French professor (my primary speciality is Spanish) if I had all of my letters in and he was very kind in offering support of my thesis, phd work, etc. (Unsure if this means he was offering to write letters if I wanted or what, but I digress.) Due to the nature of the program, I was thinking of asking if he wanted to be a fourth writer (the online app allows max 6, program site says "three"), since he is a French professor that can speak to my abilities in French and not just Spanish, and one of my writers only knows my skills in English linguistics. 

Do you think it could help or hurt the application to ask? And is it within an appropriate amount of time, or even warranted after our talk? 

Disclosure - I read fuzzylogician's post on this in the LOR section, which was helpful for sure, but since this is more language-specific I was wondering if the suggestions were different?

Thanks!

Edited November 15, 2016 by scarvesandcardigans