For words that have only three letters, there's no 4th letter. One example is with the word “insert”, there are no far more letters afterwards and so needs to be the word "add". The word “addict” is while in the BIP39 wordlist however, you would have applied “addi” for “addict”. https://hubwebsites.com/story20175598/facts-about-bip39-seed-phrase-github-revealed