For words that only have three letters, there is no 4th letter. For example with the word “include”, there aren't any a lot more letters Later on and therefore has to be the word "include". The word “addict” is in the BIP39 wordlist however , you would have used “addi” https://one-bookmark.com/story19412398/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator