For words that have only three letters, there is no 4th letter. One example is with the word “include”, there isn't any a lot more letters Later on and therefore have to be the word "include". The word “addict” is from the BIP39 wordlist however you would've used “addi” for https://www.tumblr.com/generalllamacolor/772952501127380992/opening-the-globe-of-bip39-a-comprehensive-guide