For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “include”, there are no extra letters Later on and therefore have to be the word "increase". The word “addict” is from the BIP39 wordlist however you would have utilised “addi” https://mysocialguides.com/story4203365/the-basic-principles-of-bip39-passphrase-generator