For words that have only 3 letters, there isn't any 4th letter. One example is with the word “insert”, there aren't any more letters Later on and therefore need to be the word "incorporate". The word “addict” is within the BIP39 wordlist however you would've utilised “addi” for “addict”. By https://businessbookmark.com/story4277680/a-review-of-bip39-passphrase