For words that have only 3 letters, there's no 4th letter. One example is with the word “add”, there are no extra letters Later on and therefore need to be the word "insert". The word “addict” is within the BIP39 wordlist however you would've made use of “addi” for “addict”. https://geniusbookmarks.com/story19462781/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39