lsac will not send more than 3. And schools generally require 2, and will accpet/recommend 3.
Honestly, in all likelihood, no one's going to care enough about your app to read that 4th rec. They have enough paper work and app materials to read through without you sending extras.
If you really want to put that 4th rec to good use, wait until/if you get waitlisted at a school. Sometimes sending in something different will help in deciding who gets a spot off the waitlist, and it also shows more interest in the school on your part.(don't waste your time if they all just say how great you are. Unless the rec brings somethign new to the table, it probably won't help)