For words that only have three letters, there is not any 4th letter. For example with the word “add”, there are no more letters Later on and thus must be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would have utilized “addi” for “addict”. https://bookmark-group.com/story4364966/the-basic-principles-of-bip39-passphrase-generator