For words that only have three letters, there isn't a 4th letter. By way of example with the word “include”, there won't be any more letters Later on and therefore has to be the word "add". The word “addict” is from the BIP39 wordlist however , you would've used “addi” https://hypebookmarking.com/story18588781/5-essential-elements-for-bip39-explained