Recommendation Letters for PhD application

[causalinf]

I am preparing for my application to PhD in statistics/biostats, and just asked three advisors for rec letters. I did statistics research with all three of them, one for a year and the other two for 6 months. This semester, I am taking a grad level stats class related to my research, and I have an amazing relationship with the professor teaching the class. Specifically, we discuss about the philosophical views of the field very frequently, and the professor complemented me for my passion and insights. I am thinking about asking this professor for rec letter. This means I will have total number of 4 rec letters. Is it allowed to submit 4 rec letters? If the limit is 3 letters, how should I choose which letters to include? Btw, this professor is very influential in the research field I am interested in. 

[Stat Assistant Professor]

Do not send more than 3 letters of recommendation. If you send a fourth one, the adcoms will probably only look at three of them. 

I would recommend that you send one letter of recommendation from the graduate-level statistics professor who can discuss your math/stat abilities (so you should probably tell one of your research advisors that their letter is not needed anymore)... you don't really need three letters to all come from research advisors. Having at least one from someone who has taught you and can speak to your classroom performance is a better idea.