For words that have only three letters, there is not any 4th letter. By way of example with the word “add”, there isn't any extra letters afterwards and thus needs to be the word "add". The word “addict” is during the BIP39 wordlist however , you would have used “addi” https://bookmarksystem.com/story18822551/facts-about-bip39-seed-phrase-word-list-revealed