For words that have only 3 letters, there isn't any 4th letter. One example is with the word “insert”, there won't be any extra letters Later on and therefore should be the word "add". The word “addict” is during the BIP39 wordlist however, you would've used “addi” for “addict”. A https://techonpage.com/story4882521/top-guidelines-of-bip39-word-list-pdf