For words that have only 3 letters, there is not any 4th letter. By way of example with the word “add”, there are no extra letters afterwards and so need to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've utilised “addi” for “addict”. http://andersonsaeu988.iamarrows.com/from-mnemonics-to-safety-and-security-recognizing-bip39-passphrases-and-their-significance-in-copyright