For words that only have 3 letters, there is no 4th letter. One example is with the word “include”, there isn't any additional letters afterwards and so have to be the word "add". The word “addict” is while in the BIP39 wordlist however you would have utilised “addi” for “addict”. https://bookmarks-hit.com/story19674506/bip39-seed-phrase-mnemonics-generator-an-overview