For words that only have three letters, there's no 4th letter. One example is with the word “incorporate”, there won't be any additional letters afterwards and thus should be the word "add". The word “addict” is from the BIP39 wordlist however you might have made use of “addi” for “addict”. https://ilovebookmarking.com/story18831319/getting-my-bip39-words-to-numbers-to-work