For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “insert”, there aren't any extra letters Later on and so must be the word "insert". The word “addict” is from the BIP39 wordlist however you would have utilized “addi” for “addict”. https://nowbookmarks.com/story18876074/a-secret-weapon-for-bip39-words-to-numbers