For words that only have three letters, there isn't any 4th letter. One example is with the word “insert”, there won't be any extra letters afterwards and thus need to be the word "add". The word “addict” is during the BIP39 wordlist however, you might have utilized “addi” for “addict”. https://bookmarkunit.com/story18897779/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool