For words that only have three letters, there is absolutely no 4th letter. For instance with the word “increase”, there aren't any a lot more letters afterwards and so has to be the word "increase". The word “addict” is inside the BIP39 wordlist however you would have used “addi” for https://ilovebookmarking.com/story18906439/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool