For words that only have three letters, there's no 4th letter. By way of example with the word “include”, there won't be any more letters afterwards and thus need to be the word "include". The word “addict” is during the BIP39 wordlist however, you might have used “addi” for “addict”. https://bookmarkja.com/story20870138/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator