For words that have only 3 letters, there's no 4th letter. As an example with the word “insert”, there isn't any extra letters afterwards and thus has to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would have made use of “addi” for https://seolistlinks.com/story20190813/top-guidelines-of-bip39-word-list-pdf