For words that have only 3 letters, there isn't any 4th letter. For instance with the word “insert”, there won't be any extra letters afterwards and therefore need to be the word "add". The word “addict” is while in the BIP39 wordlist however , you would have made use of https://ariabookmarks.com/story4740650/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator