For words that only have three letters, there isn't a 4th letter. By way of example with the word “include”, there isn't any extra letters afterwards and so has to be the word "add". The word “addict” is from the BIP39 wordlist however you might have used “addi” for “addict”. https://bookmarkedblog.com/story19524626/top-guidelines-of-bip39-word-list-pdf