For words that only have three letters, there's no 4th letter. As an example with the word “insert”, there isn't any additional letters afterwards and so has to be the word "include". The word “addict” is while in the BIP39 wordlist however, you might have employed “addi” for “addict”. BIP39 https://www.mediafire.com/file/qad32ryob76naol/pdf-7033-8222.pdf/file