For words that only have three letters, there is absolutely no 4th letter. For instance with the word “include”, there aren't any a lot more letters afterwards and so should be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://thesocialintro.com/story4343589/fascination-about-invalid-bip39-phrase