For words that only have three letters, there is not any 4th letter. One example is with the word “add”, there are no much more letters afterwards and therefore must be the word "insert". The word “addict” is inside the BIP39 wordlist however you would have utilised “addi” for “addict”. https://bookmarkblast.com/story18858754/bip39-phrase-generator-can-be-fun-for-anyone