FaridHadi, you are correct. Thanks so much.
In case anyone else has the same issue, here is how I resolved it: The problem was that in Settings >> Discussion I have threaded comments enabled 4 levels deep, therefore the 4th level Reply button was missing in comments. I changed the settings to allow for 5 level deep commenting and then added "display: none;" to the "reply" class for the 5th level.
Thanks again, Farid! This was driving me crazy so your help is much appreciated.