For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there isn't any extra letters afterwards and therefore need to be the word "add". The word “addict” is within the BIP39 wordlist however, you would've utilized “addi” for “addict”. By adhering to https://macrobookmarks.com/story19046847/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting