For schools that require "at least 3" letters of recommendation for a grad school application, is it smart to send 4? I am applying for math PhD and may have 4 letters of recommendation: One from my masters advisor, one from a professor I'm doing a small project with, one which I did undergrad research with and took grad algebra with, and one I took grad complex with. The last one is more recent in terms of me knowing him, but I did very well in his class. 

For schools that either say "at least 3" or do not specify, should I do all 4 letters? Do I run the risk of the committee choosing the first 3 that they see, and discarding the 4th?
Personally, having one extra (a 4th) is probably not a bad idea.  However, there are risks involved.  First, just the logistics for interacting with 3 people for getting 3 recommendations is challenging enough.  If you add a 4th person to ask for a recommendation then your work may go up by a good amount.  Second, having a 4th raises the risk of having a negative statement from a recommendor that could hurt your chances.