For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “add”, there won't be any a lot more letters Later on and therefore should be the word "increase". The word “addict” is inside the BIP39 wordlist but you might have used https://expressbookmark.com/story18903526/considerations-to-know-about-bip39-word-list-pdf