I agree: follow instructions. If anything, it's to most people's benefit to submit fewer letters.
For example: let's say you have 4 coins in your pocket (penny, nickel, dime, and quarter) and someone asks you to present 3 of these (of your choice) to give the best average impression of your wealth. Why in the world would you also show the penny? The only exception is if you have FOUR letters from VERY well-known persons and so the penny becomes a trump coin instead.
Remember that you have a choice of which letters you send. So, for instance, you're applying to a spine-heavy program and a joint-heavy program. You might, therefore, want to bias your letters for the recipient. Adding another would only serve to dilute your application.