For words that have only three letters, there isn't any 4th letter. One example is with the word “include”, there isn't any extra letters afterwards and therefore must be the word "add". The word “addict” is while in the BIP39 wordlist however you would have employed “addi” for “addict”. Mnemonic https://tetrabookmarks.com/story19638606/considerations-to-know-about-bip39-word-list-pdf