For words that only have 3 letters, there is not any 4th letter. By way of example with the word “add”, there won't be any additional letters afterwards and so has to be the word "include". The word “addict” is during the BIP39 wordlist however you would've used “addi” for https://minibookmarks.com/story19587231/the-2-minute-rule-for-bip39-all-words