For words that have only 3 letters, there's no 4th letter. By way of example with the word “add”, there are no additional letters afterwards and so has to be the word "include". The word “addict” is in the BIP39 wordlist however, you would have utilised “addi” for “addict”. In https://expressbookmark.com/story18829354/considerations-to-know-about-bip39-word-list-pdf