For words that have only 3 letters, there is not any 4th letter. One example is with the word “add”, there are no more letters Later on and therefore should be the word "incorporate". The word “addict” is during the BIP39 wordlist but you would have utilised “addi” for “addict”. https://bookmarkingace.com/story18800162/the-basic-principles-of-bip39-passphrase-generator