For words that have only three letters, there is no 4th letter. As an example with the word “incorporate”, there won't be any much more letters afterwards and thus have to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would have employed “addi” https://bookmarkquotes.com/story19050795/5-simple-techniques-for-bip39-word-list-pdf