For words that have only 3 letters, there's no 4th letter. As an example with the word “include”, there aren't any much more letters afterwards and thus has to be the word "include". The word “addict” is in the BIP39 wordlist however you would've made use of “addi” for “addict”. https://bookmarkgenius.com/story19488192/5-simple-techniques-for-bip39-seed-phrase