The maximum number of letters you can submit is 4. If you do some digging on the forum, you'll find people usually submit:
2-sciences from the pre-reqs, 1 from a dentist, and 1 non-science. I would say this is the norm. and is usually what is required, at the least.
Contact the schools you're interested in, that for instance, would like 3 science letters, like columbia, and ask them if you can have the extra science letter they need faxed or mailed to them, by the writer. Some schools require that 1 of the 2 science letters be in biology, like Tufts! So you really have to do some research based on the schools you are planning on applying to and plan accordingly, all that information is usually right on the individual schools website.
Good Luck!🙂