For words that only have three letters, there isn't a 4th letter. One example is with the word “include”, there aren't any additional letters Later on and therefore have to be the word "include". The word “addict” is in the BIP39 wordlist however, you would've utilised “addi” for “addict”. Mnemonic https://bookmarkfly.com/story18825284/top-guidelines-of-bip39-word-list-pdf