For words that have only three letters, there is not any 4th letter. One example is with the word “add”, there aren't any extra letters afterwards and therefore should be the word "add". The word “addict” is during the BIP39 wordlist however, you would have utilized “addi” for “addict”. bip39-phrase.com https://topsocialplan.com/story4308487/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition