For words that have only 3 letters, there isn't any 4th letter. As an example with the word “incorporate”, there aren't any far more letters Later on and therefore should be the word "insert". The word “addict” is within the BIP39 wordlist however, you would have applied “addi” for “addict”. https://scrapbookmarket.com/story19608455/the-2-minute-rule-for-bip39-all-words