I have 4 reference letters at my disposal, but for most jobs I need to choose 3. Here are the 4 letters… which one should NOT make the cut when I need to choose 3? And why?
A) main postdoc advisor (quite well-known, we've published multiple papers together; will write a very strong letter)
B) postdoc mentor (very well-known, we've not published together, but s/he will write a strong letter)
C) PhD advisor (relatively unknown, we only have 1 paper together, doesn't do much research these days; will write a strong letter)
D) unofficial PhD mentor (well-known, very active in my field, will write a very strong letter even though we've never actually collaborated)