For words that have only three letters, there's no 4th letter. As an example with the word “include”, there are no much more letters Later on and so have to be the word "increase". The word “addict” is during the BIP39 wordlist however you would have used “addi” for “addict”. https://bookmarkpagerank.com/story18999061/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting