For words that have only three letters, there isn't any 4th letter. As an example with the word “insert”, there isn't any far more letters afterwards and so has to be the word "insert". The word “addict” is from the BIP39 wordlist however, you would've utilised “addi” for “addict”. Security: https://wearethelist.com/story21050192/a-review-of-bip39-passphrase