For words that have only three letters, there is absolutely no 4th letter. For example with the word “insert”, there aren't any much more letters afterwards and thus should be the word "insert". The word “addict” is during the BIP39 wordlist however, you would've used “addi” for “addict”. By next https://socialdummies.com/story3836267/a-review-of-bip39-passphrase