For words that have only 3 letters, there is no 4th letter. One example is with the word “incorporate”, there aren't any more letters Later on and thus need to be the word "increase". The word “addict” is while in the BIP39 wordlist however you would've utilized “addi” for “addict”. https://iwanttobookmark.com/story19016368/top-guidelines-of-bip39-word-list-pdf