For words that only have three letters, there isn't a 4th letter. For example with the word “insert”, there aren't any much more letters afterwards and so need to be the word "include". The word “addict” is in the BIP39 wordlist but you would've applied “addi” for “addict”. Protection: Everyone https://bookmarkize.com/story18881283/the-definitive-guide-to-bip39-seed-phrase