For words that only have 3 letters, there's no 4th letter. One example is with the word “include”, there won't be any far more letters afterwards and thus must be the word "add". The word “addict” is from the BIP39 wordlist however you would've applied “addi” for “addict”. The usage https://ezmarkbookmarks.com/story19580852/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting