For words that only have three letters, there isn't any 4th letter. For example with the word “insert”, there are no extra letters Later on and therefore must be the word "insert". The word “addict” is within the BIP39 wordlist however you would've utilized “addi” for “addict”. The usage of https://socialwebconsult.com/story4310193/facts-about-bip39-seed-phrase-github-revealed