For words that only have three letters, there isn't any 4th letter. For example with the word “incorporate”, there aren't any much more letters afterwards and thus has to be the word "increase". The word “addict” is from the BIP39 wordlist however you might have employed “addi” for “addict”. This https://www.tumblr.com/generalllamacolor/772952501127380992/opening-the-globe-of-bip39-a-comprehensive-guide