For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “incorporate”, there won't be any much more letters Later on and thus needs to be the word "increase". The word “addict” is from the BIP39 wordlist but you might have https://yesbookmarks.com/story19002716/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool