For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “add”, there aren't any more letters Later on and so have to be the word "incorporate". The word “addict” is from the BIP39 wordlist however you would have used “addi” https://single-bookmark.com/story19035478/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator