For words that only have three letters, there isn't a 4th letter. For example with the word “add”, there are no additional letters afterwards and so must be the word "insert". The word “addict” is during the BIP39 wordlist however , you would have made use of “addi” for “addict”. https://bookmarkleader.com/story18891987/top-latest-five-bip39-seed-phrase-mnemonics-generator-urban-news