For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “include”, there aren't any additional letters afterwards and so has to be the word "include". The word “addict” is during the BIP39 wordlist however , you would have utilised “addi” for “addict”. https://pr6bookmark.com/story19306455/facts-about-bip39-seed-phrase-github-revealed