For words that only have three letters, there is absolutely no 4th letter. For example with the word “add”, there aren't any a lot more letters afterwards and so need to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you would have utilised “addi” https://bookmarkboom.com/story19015522/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool