For words that only have three letters, there isn't any 4th letter. One example is with the word “insert”, there aren't any extra letters afterwards and therefore has to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however, you would have used “addi” for “addict”. Security: https://bookmarksea.com/story18823740/a-secret-weapon-for-bip39-words-to-numbers