I was also wondering about the idea mix of LORs for TY/Prelim Programs. For rads, I used 2 clinical, 1 rads, 1 research advisor (>4 years). Our school sends the medicine dept. letter with the Dean's letter, although the IM chair told me it would be strong. I was thinking about the following mix:
Prelim: IM, IM, Research Advisor, Medicine Chair
TY: IM, IM, Research Advisor, (Rads or Medicine Chair?)
I thought that it would probably be remiss not to include my research advisor, since he is mentioned frequently on my app. Although I don't think prelim/TY care too much about research, I was concerned that it could raise a red flag not to include this letter and use rads instead. Would prelim programs even care if I don't include my rads letter? Do you need the medicine department letter for TYs (as a believe you do for some prelims)?