For words that have only three letters, there isn't a 4th letter. One example is with the word “incorporate”, there won't be any more letters afterwards and so need to be the word "incorporate". The word “addict” is from the BIP39 wordlist however you would have used “addi” for “addict”. https://bookmarkgenious.com/story19151009/the-invalid-bip39-phrase-diaries