Ah, nope.
If they ask for three, send three. Admissions committees don't want people sending a bunch of extra stuff in the packages. At worst, it can reflect badly on you in making it look like you didn't read the guidelines, or don't think they apply to you.
Only use doc-level people, unless you have two *kickass* doc letters and then one non-doc who knows you really well and can comment on something unique that the other letters didn't get (e.g. clinical supervisor or whatever).
Didn't seem to hurt my number of interviews. While in theory it could hurt your chances, it did not hurt mine. I talked to a number of the programs and some said they would only use the 1st three letters, but understood that I was having 4 sent just to make sure that my packets were complete on time and that one letter was there as a backup simply because things happen and letters can get lost in the process. Either by you, your faculty, the post office, their mail room, accidentally filed wrong, etc, etc.
I do not believe that I was denied by any program because of this. Over 14 of the 27 programs I applied to offered me an interview (more than 50%). Some programs that I did not get interviews from were simply poor matches, did not have faculty that matched my interests taking students that year, or had better candidates than me.
I applied to some wickedly competitive programs... I applied to some lessor programs. At both ends of the spectrum I had problems. I did very well in the middle upper range and where, honestly, my scores indicated I should be applying. Schools looking for people in the 1000-1100 range GREs were less likely to interview me. School looking for 1350-1450 GREs and Ivy educations ignored me as well.
I learned alot during my unsuccessful year and the time spent licking my wounds to get into a good program. I accomplished my goal, all I can do is relate my experiences. They certainly are not gospel, but they hopefully are helpful data points. Maybe someone here has been turned down for giving an extra letter of recommendation... if so, I hope they share their story with us.
I will leave you with this:
I submitted an essay to Texas A&M and put the professors from Texas Tech in the letter. I wrote off A&M figuring I had 0 chance at admission. After all who would admit a student after such a huge gaffe? I was admitted to the program, sight unseen, with a phone interview on January 15th!!!
You never know where the chips are gonna fall!
Mark