For words that only have three letters, there is absolutely no 4th letter. As an example with the word “add”, there isn't any a lot more letters Later on and thus needs to be the word "insert". The word “addict” is within the BIP39 wordlist however, you would have utilised https://linkedbookmarker.com/story4348243/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator