For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “insert”, there won't be any much more letters afterwards and thus need to be the word "include". The word “addict” is while in the BIP39 wordlist however , you might have employed https://olivebookmarks.com/story18911464/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator