For words that have only 3 letters, there isn't any 4th letter. Such as with the word “insert”, there won't be any a lot more letters afterwards and so needs to be the word "insert". The word “addict” is within the BIP39 wordlist however, you would've used “addi” for “addict”. https://getsocialpr.com/story19847949/5-essential-elements-for-bip39-explained