For words that only have 3 letters, there is no 4th letter. For example with the word “incorporate”, there are no extra letters Later on and therefore needs to be the word "insert". The word “addict” is in the BIP39 wordlist however , you would have applied “addi” for “addict”. https://express-page.com/story4867091/little-known-facts-about-bip39-words