For words that only have three letters, there is not any 4th letter. As an example with the word “include”, there isn't any extra letters afterwards and therefore has to be the word "insert". The word “addict” is while in the BIP39 wordlist however, you would have made use of https://guidemysocial.com/story4434395/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator