For words that have only three letters, there's no 4th letter. For instance with the word “include”, there aren't any much more letters afterwards and thus need to be the word "include". The word “addict” is in the BIP39 wordlist but you would have used “addi” for “addict”. Mnemonic words https://bookmarketmaven.com/story19447621/facts-about-bip39-seed-phrase-word-list-revealed