I will add my nifty >4 letters/program trick here again. You can upload as many LORs to ERAS as you wish. I personally had 6 different ones up there, 3 clinical, 2 research and 1 *******, useless chairman's letter that IM programs insist on having. But I digress.
If you wish to transmit > 4 letters to a program, here's what you do. Go ahead and transmit 3 or 4, however many you want to a program. Then, when they've downloaded them (and ERAS will tell you when each program has downloaded each part of your app), go back in and change assignments to that particular program so that 1 or 2 of the letters you already sent (and which are now hard copies or local machine copies at each program) are no longer being sent and add 1 or 2 letters. Most of the time, these will get transmitted. I actually saw 5 letters in my file at 2 different interviews, a 3rd program interviewer asked me how I got 5 letters in my file and I checked in with 3 other programs to make sure they'd gotten my last transmitted LOR.
I found this out by a fluke. I waited and waited for one of my LOR writers to get it together and submit my letter and, by late October, it hadn't happened. So I panicked and transmitted a different (still good but not phenomenal) letter that I'd been saving in reserve. The next day, the phenomenal letter got uploaded to ERAS and I did the switch I described above. I can't say for sure that all my programs got it but enough of them (including my top 5 programs) did that I was happy with the outcome.
YMMV of course. I saw that some programs never dl'd the late letter at all and I didn't check with every program to see if they got all 5 letters.
Good luck,
BE (now PE)