For words that have only three letters, there isn't a 4th letter. For instance with the word “include”, there aren't any more letters afterwards and thus have to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would've used “addi” for “addict”. bip39-phrase.com is https://socialevity.com/story20983878/phrases-with-bit-an-overview