I think the issue here with how many letters to send deals with unclear wording. I can understand not sending an extra letter to programs who say, "Only 3 letters of rec." But, most say, "you need to following for a complete application, x, y, z, 3 letters of rec, etc." I take that to mean, in order to have a complete application, they require at least 3 letters. Since ERAS allows you to send 4, where is the harm in sending an extra one?