For words that have only 3 letters, there isn't a 4th letter. As an example with the word “increase”, there aren't any a lot more letters afterwards and so should be the word "include". The word “addict” is during the BIP39 wordlist however , you would've utilized “addi” for “addict”. https://agendabookmarks.com/story19532855/top-guidelines-of-bip39-word-list-pdf