For words that only have three letters, there is not any 4th letter. Such as with the word “add”, there aren't any much more letters afterwards and so need to be the word "add". The word “addict” is from the BIP39 wordlist however, you would've utilized “addi” for “addict”. To https://leftbookmarks.com/story19094896/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39