How to tell fourth letter writer I don't need them to write for me

[kingduck]

I initially thought I'd have trouble securing three faculty members for strong letters, so I contacted a fourth. This professor was very enthusiastic and offered to write after I sent her some of my application material like deadlines and SOPs. Well, my three letter writers came through, third one submitted almost half of his letters, and committed to finishing the rest of them. 

Should I keep fourth writer in the loop, in case my final recommender doesn't finish all of them? If not, what is the best way to tell them that I don't need anymore letters? Normally, I'd just submit all 4 letters, but many programs only want no more than 3. I could say that my final recommender who hasn't finished all the letters yet, would write me a much stronger letter.

I don't mean to sound unappreciative or as if I am using these professors, but I do want to make sure that I don't burn bridges while also having the best letters I can get.  

[cyberwulf]

No need to be fancy, just tell them you won't be needing a letter from them because you have a sufficient number from other faculty. Chances are, they'll appreciate that you're taking something off their plate.