For words that have only 3 letters, there's no 4th letter. For example with the word “insert”, there are no much more letters afterwards and thus must be the word "add". The word “addict” is during the BIP39 wordlist however, you would've utilized “addi” for “addict”. One more benefit of https://getidealist.com/story20581054/the-basic-principles-of-bip39-passphrase-generator