For words that only have three letters, there is not any 4th letter. By way of example with the word “insert”, there aren't any additional letters afterwards and therefore should be the word "add". The word “addict” is during the BIP39 wordlist however you might have made use of “addi” https://bookmarkyourpage.com/story4203014/5-simple-techniques-for-bip39-word-list-pdf