I'm applying for graduate school in a hard science this year, and most things on my application are pretty good so far. Two of my letters should be strong, but regarding the third one, the professor told me in the interest of full disclosure, that his letter will be a positive one, but certainly not the best he's written. This particular researcher is well known in his subfield, which isn't the field I'm applying to, but it's not very far away either. I've taken many of his graduate courses, and also did an independent reading course with him. The graduate courses went pretty well, but the reading course didn't unfortunately, (mostly due to a difference in style) and as a result of all this interaction, he knows me well, and he thinks I'm a good but not great student. Now I'm really hoping I can get into at least one of the better schools on my list, and it's insanely competitive, particularly in my subfield.
Another option of mine is asking another professor who doesn't know me that well and is in mathematics instead than the hard science I'm applying to, but he certainly thinks I'm a top student, since he was responsible for many of the math awards I got and I did really well in the one class that I did take with him.
Q: Would having two great and one good letter hurt me at the top places? My impression is that students who get admitted to top places have their professors say that they're the best student they've seen in many years, and not having such a letter could result in rejection. Which recommendation should I go for? Is it advisable that I even do both and send in four letters?