For words that only have three letters, there isn't any 4th letter. One example is with the word “insert”, there won't be any a lot more letters afterwards and thus must be the word "increase". The word “addict” is from the BIP39 wordlist however you would've applied “addi” for “addict”. https://dmozbookmark.com/story19018826/the-basic-principles-of-bip39-passphrase-generator