For words that only have three letters, there isn't any 4th letter. Such as with the word “include”, there won't be any more letters afterwards and thus need to be the word "insert". The word “addict” is in the BIP39 wordlist however, you would have employed “addi” for “addict”. Standard https://thegreatbookmark.com/story19066416/new-step-by-step-map-for-bip39-english-words