For words that have only 3 letters, there isn't a 4th letter. One example is with the word “insert”, there aren't any much more letters afterwards and so need to be the word "insert". The word “addict” is from the BIP39 wordlist however , you would've made use of “addi” https://bookmarkwuzz.com/story18793926/bip39-seed-phrase-fundamentals-explained