For words that have only 3 letters, there's no 4th letter. For instance with the word “add”, there isn't any a lot more letters afterwards and therefore has to be the word "increase". The word “addict” is within the BIP39 wordlist however you would've employed “addi” for “addict”. In this https://e-bookmarks.com/story4452923/facts-about-bip39-seed-phrase-word-list-revealed