For words that have only 3 letters, there isn't any 4th letter. One example is with the word “insert”, there are no extra letters Later on and so needs to be the word "include". The word “addict” is in the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://gogogobookmarks.com/story18862469/the-invalid-bip39-phrase-diaries