Some schools want 3 science LORs, some want 2 + 1 LOR from dentist, or some want 1 bio + 1 chem LOR, it is all really dependent on the schools you are applying to.
I suggest researching schools you are interested in, finding out what they want, and compiling a list of LORs to get. On the application, you can only have 4. But, you can send in additional LORs directly to the school. But again, I would check on dental schools, call them even to get a clear answer in what is allowed and not allowed.