For words that only have 3 letters, there is no 4th letter. One example is with the word “add”, there are no far more letters afterwards and so has to be the word "insert". The word “addict” is in the BIP39 wordlist however you would have utilised “addi” for “addict”. https://madbookmarks.com/story19471621/considerations-to-know-about-bip39-word-list-pdf