For words that have only 3 letters, there is not any 4th letter. By way of example with the word “insert”, there aren't any additional letters afterwards and thus has to be the word "include". The word “addict” is while in the BIP39 wordlist however , you would've applied “addi” https://bookmarklethq.com/story18811596/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting