For words that only have 3 letters, there is no 4th letter. One example is with the word “insert”, there aren't any extra letters afterwards and so have to be the word "include". The word “addict” is in the BIP39 wordlist however, you would've employed “addi” for “addict”. The use https://www.mediafire.com/file/qad32ryob76naol/pdf-7033-8222.pdf/file