For words that only have 3 letters, there's no 4th letter. By way of example with the word “insert”, there won't be any much more letters Later on and thus has to be the word "insert". The word “addict” is within the BIP39 wordlist however , you would've used “addi” http://jaspermuxp089.yousher.com/from-mnemonics-to-security-comprehending-bip39-passphrases-and-their-significance-in-copyright