My 4 LORs came from 2 science professors +1 dentist + 1 employer from undergrad. I worked at one of the dorms under my boss for 4 years, and even though my job wasn't related to the health field at all, I strongly believed he could speak on many of my non-academic factors and development as a leader. As someone above already mentioned, whichever LOR combination gives you the best overall picture of you, go with that one! I could've asked another professor from my post-bac to write my last letter, but he's super busy and picky with whom we writes LOR's for, and I wanted to present myself in a non-academic manner too. Hope this helps!