For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “add”, there are no additional letters afterwards and therefore should be the word "insert". The word “addict” is from the BIP39 wordlist however you would've used “addi” for “addict”. If https://peakbookmarks.com/story18986043/considerations-to-know-about-bip39-word-list-pdf