For words that only have three letters, there's no 4th letter. One example is with the word “include”, there aren't any much more letters Later on and thus needs to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've made use of “addi” for https://yourbookmarklist.com/story18972087/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition