For words that have only 3 letters, there isn't any 4th letter. One example is with the word “insert”, there won't be any extra letters afterwards and thus needs to be the word "increase". The word “addict” is from the BIP39 wordlist however , you would've applied “addi” for “addict”. http://jaspermuxp089.yousher.com/from-mnemonics-to-security-comprehending-bip39-passphrases-and-their-significance-in-copyright