For words that only have 3 letters, there is no 4th letter. One example is with the word “include”, there aren't any much more letters afterwards and thus should be the word "add". The word “addict” is from the BIP39 wordlist however, you would have utilised “addi” for “addict”. - https://single-bookmark.com/story18872998/getting-my-bip39-words-to-numbers-to-work