For words that have only 3 letters, there is no 4th letter. By way of example with the word “increase”, there are no extra letters Later on and so have to be the word "increase". The word “addict” is from the BIP39 wordlist however you would have applied “addi” for https://bookmarkwuzz.com/story19601909/the-basic-principles-of-bip39-passphrase-generator