I absolutely echo what was said before about quality of the comments vs. the source. BTW For my prelim I used my internal medicine letter (since I matched into IM). Though I submitted 2 PM&R letters, they were strong, but somewhat generic I think (since one of my programs essentially read excerpts from them to me during my interview day and I was able to figure out who was who). The letter that came through the most for me and the one that kept on being brought up during interviews was a letter from a neurologist at my school; neurology had just been made a core clerkship and I took it seriously (unlike many of my classmates) and in the end I'm pretty sure THAT letter is what got me my interviews and what helped my ranking within programs.
You never know who will be your best advocate, and you can upload multiple letters and choose which four you want to submit to different programs. My advice is to make a good impression on everyone, and work hard, so that you know you will receive multiple strong letters, and be able to choose which ones make it to the Final Four (lol)
