For words that only have 3 letters, there isn't any 4th letter. For instance with the word “include”, there aren't any additional letters afterwards and so has to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you might have employed “addi” for “addict”. https://bookmarkinglife.com/story5030545/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator