For words that only have 3 letters, there's no 4th letter. By way of example with the word “insert”, there won't be any far more letters afterwards and so have to be the word "insert". The word “addict” is inside the BIP39 wordlist however you would have made use of https://bookmarkunit.com/story18674834/facts-about-bip39-seed-phrase-word-list-revealed