For words that only have 3 letters, there isn't any 4th letter. For instance with the word “insert”, there won't be any more letters Later on and thus need to be the word "add". The word “addict” is in the BIP39 wordlist however , you would've made use of “addi” http://josueaqpi312.huicopper.com/from-words-to-riches-just-how-to-successfully-make-use-of-bip39-passphrases-for-your-copyright-budgets