For words that only have 3 letters, there is not any 4th letter. As an example with the word “insert”, there are no far more letters Later on and so need to be the word "include". The word “addict” is within the BIP39 wordlist however, you might have made use https://ilovebookmarking.com/story19470532/little-known-facts-about-bip39-seed-phrase-list