For words that have only three letters, there is no 4th letter. For example with the word “incorporate”, there isn't any much more letters Later on and so has to be the word "include". The word “addict” is within the BIP39 wordlist however you would have utilized “addi” for “addict”. https://bookmarklinking.com/story6303518/5-essential-elements-for-bip39-explained