I currently have 3 LOR, 1 science, 1 non-science, and a dentist I shadowed. My 4th letter is supposed to come from another science professor but he said to contact him in mid July because he's very busy now (I asked him in early May). Can I submit my application with the 3 letters I have now and just add him later or do all 4 emails have to be added before I submit? I don't want to add his email until I have full confirmation he'll do it.