For words that have only 3 letters, there isn't a 4th letter. As an example with the word “add”, there aren't any a lot more letters afterwards and therefore should be the word "include". The word “addict” is while in the BIP39 wordlist however you would've used “addi” for “addict”. https://pr7bookmark.com/story19029371/5-simple-techniques-for-bip39-seed-phrase