For words that have only 3 letters, there isn't any 4th letter. Such as with the word “incorporate”, there are no far more letters Later on and thus have to be the word "include". The word “addict” is from the BIP39 wordlist however you would have applied “addi” for “addict”. https://iowa-bookmarks.com/story14614911/5-essential-elements-for-bip39-explained