For words that have only 3 letters, there's no 4th letter. By way of example with the word “insert”, there aren't any a lot more letters Later on and therefore needs to be the word "include". The word “addict” is in the BIP39 wordlist however you would have used “addi” https://www.slideserve.com/lewarthbfp/bip39-word-list-explained-your-essential-source-for-creating-secure-purses-in-t