For words that have only three letters, there is no 4th letter. Such as with the word “include”, there aren't any much more letters Later on and therefore should be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have made use of “addi” https://www.tumblr.com/generalllamacolor/772952501127380992/opening-the-globe-of-bip39-a-comprehensive-guide