For words that only have 3 letters, there is no 4th letter. For instance with the word “include”, there are no a lot more letters afterwards and thus have to be the word "include". The word “addict” is within the BIP39 wordlist however you would have utilised “addi” for “addict”. https://getidealist.com/story20916087/fascination-about-bip39-english-words