For words that have only 3 letters, there isn't a 4th letter. One example is with the word “incorporate”, there aren't any extra letters Later on and thus must be the word "insert". The word “addict” is within the BIP39 wordlist however , you would have applied “addi” for “addict”. https://reallivesocial.com/story4507743/a-review-of-bip39