For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “add”, there are no additional letters Later on and thus needs to be the word "add". The word “addict” is inside the BIP39 wordlist however , you would have utilized “addi” for https://johsocial.com/story9237367/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool