Letters of Recommendation - Advice appreciated!!

[factanonverba]

Hi guys,

I would love to hear some advice from you about who to ask for letter of recommendation. 

About Me: I worked a year in an undergrad research lab, and I started working in my current lab after I graduated 2 years ago. I'm applying to biomedical PhD programs soon for the 2018 admission cycle (Harvard, MIT, Stanford, Yale, etc.)

My dilemma: Most of the programs I am going to apply to ask for three letters of recommendation, while there are four people that I want to ask!

1. My undergrad research PI, who was also my biochemistry professor - I think this makes sense because he could speak to my performance in a class environment as well as a research environment, although I stopped working with him two years ago and since then my research abilities have greatly enhanced, so he probably does not have the most up-to-date information about me.

2. My current research PI - He definitely makes sense because he is a big name in the immunology/immunotherapy field, and I have worked for him for 2 years now. During this time, I started a research project from the ground up and wrote a manuscript as the primary author; this project has now also gained a lot of industrial interest, so it is rapidly expanding in scope. Therefore, I think he would be able to provide quite strong recommendation for me.

3. Another PI - he is relatively junior (assistant professor), but still a faculty member. I have worked with him very closely in the past year when I was writing my manuscript. I believe he can speak well to my research abilities.

4. My immediate supervisor in my current lab (non faculty member) - I work with her closely and she can certainly speak about my research abilities and personality/work ethic. I would say she knows me quite well personally.

So I think #1 and #2 are obvious choices, but I am struggling to choose between #3 and #4. #3 is a PI/faculty member so his letter might carry more weight? He is a junior faculty member though so I don't know how much of a difference it makes. #4 is my current supervisor and she thinks quite highly of me. However, she is not a PI/faculty member so is this less ideal? I would feel a bit weird though bypassing her and asking #3 for recommendation. What do you guys think?

[Bioenchilada]

Drop the last person. You already have your current PI giving you a rec letter, no need to get another from the same lab. She'll also probably communicate with your PI about your abilities/personalty if help is needed when witing the letter. Also, adding more than 3 letters seldom helps, don't make the adcom read more than they have to in order to make a decision about an interview/offer.

[TakeruK]

Definitely use the first three. Let the 4th person know that you are applying and asking #2 (current research PI) for a letter. Ask #4 if she would be willing to talk to #2 about your work since you work most closely with her. Assuming she says yes, when you ask #2 for the letter, you can say that you talked about your application with #4 and that #4 could provide more information about your work if #2 wants/needs it.

Also agree with only submitting 3 letters unless an application specifically asks for four.

[factanonverba]

Thanks Bioenchilada and TakeruK! These suggestions are helpful.