For words that have only 3 letters, there is absolutely no 4th letter. By way of example with the word “increase”, there aren't any a lot more letters afterwards and thus should be the word "increase". The word “addict” is in the BIP39 wordlist however, you would have made use https://admiralbookmarks.com/story18832783/a-secret-weapon-for-bip39-words-to-numbers