For words that only have three letters, there is no 4th letter. By way of example with the word “add”, there aren't any extra letters Later on and thus have to be the word "insert". The word “addict” is during the BIP39 wordlist however you would've utilised “addi” for “addict”. https://baidubookmark.com/story18701290/5-simple-techniques-for-bip39-word-list-pdf