For words that have only 3 letters, there isn't any 4th letter. For example with the word “insert”, there isn't any more letters afterwards and thus has to be the word "add". The word “addict” is during the BIP39 wordlist however, you would have utilised “addi” for “addict”. Mnemonic words https://yourbookmarklist.com/story19022681/5-essential-elements-for-bip39-explained