For words that have only 3 letters, there isn't any 4th letter. Such as with the word “include”, there won't be any additional letters afterwards and so should be the word "insert". The word “addict” is in the BIP39 wordlist however you would have employed “addi” for “addict”. The usage https://bookmarkedblog.com/story19540231/fascination-about-bip39-english-words