For words that only have three letters, there isn't any 4th letter. One example is with the word “insert”, there are no additional letters afterwards and therefore must be the word "include". The word “addict” is from the BIP39 wordlist however, you would've utilized “addi” for “addict”. BIP39 and 44, https://techonpage.com/story4547669/the-2-minute-rule-for-bip39-all-words