For words that have only 3 letters, there is not any 4th letter. One example is with the word “add”, there won't be any far more letters Later on and thus has to be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would've used “addi” https://maximusbookmarks.com/story18951232/the-2-minute-rule-for-bip39-all-words