For words that only have 3 letters, there is not any 4th letter. By way of example with the word “add”, there isn't any far 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://bookmarkssocial.com/story18847069/the-2-minute-rule-for-bip39-all-words