For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “include”, there aren't any a lot more letters Later on and therefore should be the word "incorporate". The word “addict” is from the BIP39 wordlist however , you would have used “addi” https://peakbookmarks.com/story18911663/5-essential-elements-for-bip39-explained