For words that have only 3 letters, there's no 4th letter. By way of example with the word “incorporate”, there won't be any far more letters afterwards and so need to be the word "include". The word “addict” is from the BIP39 wordlist but you would've employed “addi” for “addict”. http://hectorswwo172.bearsfanteamshop.com/understanding-your-copyright-security-the-crucial-guide-to-bip39-passphrases-and-mnemonics