For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “add”, there are no a lot more letters afterwards and so need to be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you might have used https://my-social-box.com/story5031591/new-step-by-step-map-for-bip39-english-words