For words that only have three letters, there's no 4th letter. One example is with the word “insert”, there won't be any more letters afterwards and so needs to be the word "increase". The word “addict” is while in the BIP39 wordlist but you might have utilised “addi” for “addict”. http://josueaqpi312.huicopper.com/from-words-to-riches-just-how-to-successfully-make-use-of-bip39-passphrases-for-your-copyright-budgets