For words that only have 3 letters, there isn't a 4th letter. One example is with the word “add”, there are no additional letters Later on and therefore need to be the word "add". The word “addict” is within the BIP39 wordlist but you would've used “addi” for “addict”. Protection: https://bookmarksystem.com/story18642725/5-simple-techniques-for-bip39-seed-phrase