For words that only have three letters, there is not any 4th letter. By way of example with the word “increase”, there aren't any more letters afterwards and thus should be the word "increase". The word “addict” is while in the BIP39 wordlist however, you might have made use of https://peakbookmarks.com/story18885484/the-2-minute-rule-for-bip39-all-words