Should I revoke my recommendation request for a particular letter? Is this impolite? I am in a bit of a bind. I've been taking a reading course with a professor, and at first it was going fairly well. After a while (when the material got hard and I got busier), the readings were just going okay, meaning that I wasn't progressing very fast, and had trouble with almost all exercises. 

I feel that I don't impress this professor as much as I might have in the beginning. I am, however, doing well in his class currently, but not the best student I'd say. I have asked him a while ago if he'd write me a letter of recommendation for graduate school (my apps will be completed in a few weeks). I have even sent requests in the online applications for his letter. But I am thinking of revoking the requests online (this is possible). I asked him if his letter would be strong, and his reaction was not very clear. He said he'd mostly talk about how I'm doing in his topics course, and some of what we've been reading.

I have three other letter writers, all of which should be very good. Two of these professors I've done research with, the other I am in his course, but he says my solutions are very good. Now, the latter guy does not know me as well as the professor in question, but his letter, he says, will be strong based on my course work. 

So my question: Should I cancel the letter request? Is this weird to do? Should I talk to him first (if so, how?)? 

This is my future career, so dealing with awkwardness and animosity may be worth it in the end. I should also say that the readings we are doing I am no longer interested in - they were more of a test-the-waters thing for me to see if I wanted to do research in that area. 

edit. I've decided to live with my mistake and move on. Hopefully the letter isn't too bad.
No, it would not be very polite. However, I do not understand why would you cancel the request. You can just have the letter and not use it. You are not obliged to send every single reference letter to the institution you apply.

If you feel like you must cancel the request, you could tell the professor that the institution you are applying requires three letters and you already have them. Do not forget to apologize that it is your mistake that you did not carefully read the instructions.