For words that have only three letters, there isn't a 4th letter. For instance with the word “incorporate”, there aren't any more letters afterwards and therefore need to be the word "add". The word “addict” is within the BIP39 wordlist however you would have made use of “addi” for “addict”. https://bookmark-share.com/story18879335/facts-about-bip39-seed-phrase-github-revealed