For words that only have three letters, there isn't a 4th letter. One example is with the word “include”, there aren't any more letters Later on and so have to be the word "include". The word “addict” is in the BIP39 wordlist however you would've utilized “addi” for “addict”. Mnemonic https://kbookmarking.com/story18791215/fascination-about-invalid-bip39-phrase