For words that only have 3 letters, there isn't any 4th letter. As an example with the word “increase”, there isn't any extra letters Later on and therefore has to be the word "insert". The word “addict” is while in the BIP39 wordlist however you would have utilised “addi” for https://www.mediafire.com/file/qad32ryob76naol/pdf-7033-8222.pdf/file