For words that only have 3 letters, there is absolutely no 4th letter. By way of example with the word “add”, there won't be any a lot more letters afterwards and thus should be the word "add". The word “addict” is in the BIP39 wordlist however you would have applied https://hypebookmarking.com/story19254751/new-step-by-step-map-for-bip39-english-words