What kind of incident would prompt four months of additional time to be added to his release date

That is a terrible story, we are sorry for your loss, the fact that he has an out date at all is crazy. Our guess here is that it was a minor incident report that took some of his granted "good time" (all inmates get their good time credits at the beginning of their bid) which could be a short list of possible violations (fighting, contraband, gambling, possession of drugs or tobacco, or repeat violations) OR it is more likely a miscaculation of his orginal release date - and they've adjusted it.