For words that have only three letters, there's no 4th letter. For instance with the word “insert”, there won't be any extra letters afterwards and thus needs to be the word "include". The word “addict” is in the BIP39 wordlist however you would've employed “addi” for “addict”. BIP39 and 44, https://maroonbookmarks.com/story18765322/new-step-by-step-map-for-bip39-english-words