For words that have only 3 letters, there is not any 4th letter. One example is with the word “include”, there isn't any additional letters afterwards and therefore must be the word "insert". The word “addict” is within the BIP39 wordlist however , you would have utilised “addi” for “addict”. https://bookmarkpressure.com/story18734304/top-guidelines-of-bip39-word-list-pdf