For words that have only three letters, there isn't a 4th letter. For instance with the word “insert”, there aren't any extra letters afterwards and thus must be the word "increase". The word “addict” is inside the BIP39 wordlist however you would have used “addi” for “addict”. This additional complexity https://bookmarkfox.com/story5047488/5-essential-elements-for-bip39-explained