Hey, I think the problem lies in AADSAS only letting you submit 4 LORs total. I know you can individually send letters to each school, but in your 4 allocated ones, you want to make sure that they're the best they can be. I had 2 from my bcp instructors (one was a tenured prof, other was a PhD student who was just about done with his PhD), and then 1 was from a general dentist I shadowed, and the final one was from an advisor in my class, who told me on the day of my graduation he wanted to write an LOR for me, hah. Though he was only an advisor for me, he was a professor for classes in the graduate health sciences and he knew me quite well.
For other schools, I had to get letters written and sent specifically from supervisors of volunteering activities, but only a couple of schools wanted those. Unless you do a committee letter, you can't really submit that many LORs I think. It's good that all of you are starting on this now! Some poor, fellow SDN-er said their prof didn't get his letter in for months...thereby delaying his application. Good luck, and get them done ASAP!