For words that only have 3 letters, there's no 4th letter. One example is with the word “add”, there are no far more letters Later on and therefore has to be the word "add". The word “addict” is while in the BIP39 wordlist however, you might have employed “addi” for https://socialbuzzfeed.com/story4267193/5-essential-elements-for-bip39-explained