For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “add”, there aren't any much more letters afterwards and so should be the word "increase". The word “addict” is during the BIP39 wordlist however , you might have made use of “addi” https://peakbookmarks.com/story19105800/the-basic-principles-of-bip39-passphrase-generator