For words that have only 3 letters, there's no 4th letter. One example is with the word “add”, there aren't any a lot more letters afterwards and thus should be the word "add". The word “addict” is while in the BIP39 wordlist however, you might have made use of “addi” https://yourbookmarklist.com/story19783949/5-simple-techniques-for-bip39-seed-phrase