For words that have only 3 letters, there is absolutely no 4th letter. For example with the word “incorporate”, there isn't any extra letters afterwards and thus need to be the word "include". The word “addict” is within the BIP39 wordlist however , you would have applied “addi” for “addict”. https://madesocials.com/story4415632/a-secret-weapon-for-bip39-words-to-numbers