For words that have only three letters, there is absolutely no 4th letter. One example is with the word “include”, there aren't any additional letters afterwards and thus should be the word "insert". The word “addict” is within the BIP39 wordlist however, you would've utilised “addi” for “addict”. BIP39 and https://bookmarkprobe.com/story19111027/the-ultimate-guide-to-bip39-seed-phrase-github