For words that only have 3 letters, there isn't any 4th letter. For instance with the word “add”, there isn't any extra letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist however, you would have employed “addi” for “addict”. A straightforward https://wise-social.com/story4337477/the-invalid-bip39-phrase-diaries