For words that have only 3 letters, there is not any 4th letter. By way of example with the word “insert”, there won't be any much more letters Later on and so must be the word "add". The word “addict” is in the BIP39 wordlist however, you would have applied https://bookmarkchamp.com/story19548014/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition